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.