I'm not sure the drop in your Nimbus but I am sure it isn't 10mm like the Nikes. I am now Nike all in training in the Pegasus Turbo and racing in the Vaporfly both 10mm and it took me a little while to adjust to that much drop coming from Hoka. You may be ok but personally I would give myself longer to race in a shoe if I was making what is quite a major change and you could risk injury.
Edit...
I stand corrected did some googling and I see and am somewhat surprised the Nimbus 21 is stated as a 10mm drop. That being the case if they feel ok in training then why not.

I got a pair of new balance beacon’s two weeks before HIM Chattanooga. I had a couple shot runs and a 6 mile run. Everything felt fine so I wore them on race day with no issues.

Nimbus 21 is 10mm. Most cushioned shoes like Glycerin, Ultraboost, 1080, Wave Creation, and Triumph are all in the 8-12mm range with most of them at 10mm. Your Hoka is more of the outliers in that regard.

Edit: I see your edit now

I ran Indian Wells in Vapor 4% that I had only ran in twice. No issues whatsoever. Perhaps you can wear them around casually to break them in and if you have no discomfort, go for it.

Heck Yeah! Thats why you bought them! Good luck and ZOOM ZOOM :)

Honestly, at this point, I'd stick with what you know assuming this is your A race. For one, I don't particularly like running in "fast" shoes at slower paces, and please take no offense, but 9:09 is recovery run pace for the runners these shoes were designed for. I actually just got a pair of Vaporflys (slightly different I know) and took them out for a shortish tempo test run. The shoe just didn't feel right during my warm up (~8:30 pace) and it wasn't until I got into the low 7:00s that I felt like the shoe really started to come to life. I think you'd be more comfortable and just as fast in something soft but light like an Epic React at >9:00/mile.

Don't doubt in yourself and your training. Use what you know works. What if you find yourself at mile 10 of the half marathon and find your toes are chaffing badly or some old injury starts flaring up and your pace slows? That's not the time to be doubting yourself and some last minute equipment change.

Test equipment at B/C races. Now if this is a B race for you. Then I'd say (caveat above aside), take them for perhaps 2 runs (a short RP brick run, and a 3-5 mile run with some RP in there) and see how they feel. If you like them, give it a go.

Good luck on your race.

yes I think you are right. They just feel so fast. First run and instant 5k pb... 24:22

thanks !

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http://www.tri-to-improve.com

I agree with this. For me the 4% feel ok at 8:00/mi, good at 7:15/mi, and amazing below 6:30/mi. If I plan to run slower than 7:30 at a race, I prefer React or Ultraboost.

A couple of runs should be fine for breaking them in. Use them in the him...that’s why you bought them.

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ECMGN Therapy Silicon Valley:
Depression, Neurocognitive problems, Dementias (Testing and Evaluation), Trauma and PTSD, Traumatic Brain Injury (TBI)

5:20/ km is my racepace hopefully for an OD in a few months. You saying these shoes will not work for me?

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They work at any pace, but they really shine the faster you go. I don't feel much of a benefit as I slow down. They aren't very stable, so when there's little pace benefit then I prefer something more robust. Note, I'm talking about the 4% because I haven't tried my Zoom FK yet, which uses React foam. I like my Odyssey React, so it's possible the Zoom FK is better suited for all paces than the 4%.

I think BigBoyND and I are both talking about the Vaporfly. I recently ordered a pair of the Zoom Flys (with the plan to do my speedwork training runs in the Zoom Fly and race in the Vaporfly), but haven't worn the Zoom Fly yet. The Vaporfly really seems to shine the faster you go, with ~7:00/mile where it starts to come to life for me. Much slower than around 8:00/mile and I find the shoe too squishy and unstable and it messes up my form. But that's just me, and I've also only worn it twice to test it before a B-Race Olympic this weekend where I'd like to be running sub-7:00 miles, so I expect they'll feel pretty good, and I'll make an assessment if I want to wear them for my upcoming 70.3s or not. Plus you're not even asking about the Vaporfly.... so this is all really a useless digression.

My understanding is the Zoom Fly is mostly React foam, which I expect will feel fine at slower paces. I've recently started rotating in Epic Reacts for my easy/recovery runs (8:30-9:15/mile) and they feel great at that speed, and they seem to respond pretty well to faster paces as well. I'm not sure how the Zoom Fly will feel at 9:09 pace with the carbon fiber plate. It might feel like of weird like the Vaporfly, as it seems to shift me forward which makes me feel like I should be RUNNING as opposed to JOGGING.

The Zoom Fly is not an especially lightweight shoe. The Epic React a couple oz lighter and really designed as a lightweight cushion trainer that can go for miles. At the paces I expect you're running at, I'd consider racing in the Epic React before the Zoom Fly. It's a really plush and light shoe. I think it will give you the support you need and the cushion you want, while still feeling "light" and fast compared to a normal trainer.

What are you trying to accomplish by mixing in a new, sort of random shoe 1.5 weeks before your A race? (I presume it's an A race as from what I can tell you're traveling internationally for it)? Honestly, I don't think the Zoom Fly is going to make you faster. The Vaporfly might, though I still think it feels weird at slow paces. All I think it does is introduce the risk of something new, that isn't battle tested and trusted on race day. The Zoom Fly has a reputation of giving people Achilles problems. What if you get bad Achilles pain mid race? It might even make you slower, as your body is trained to be most efficient running in the shoes you've been training in. For all you know, the Zoom Fly and it's carbon plate will change your gait in ways that are not optimal for running at a 9:00/mile pace, or change your gait in ways your body has not adapted to be maximally efficient.

I don't know... I don't get the idea of throwing in a new, kind of random shoe into your race place <2 weeks from your race. I don't want to damage your confidence, so if you decide to wear them, just go for it and don't look back. But my vote is to put your trust in what you know works. There will be other races that matter less and future seasons where you can play around with new gear to try to eek out another "free" 1-2% running efficiency.