I wouldn't let that mechanic touch my bike.

Your short cage RD can handle a 32t sprocket.
Shimano would "spec" it for 28t, and in practice it's used on 32t daily. Your "formal" reference is Leonard Zinn. And for clarity, what you refer to as a "long cage", is on fact a mid length cage. A long cage would be what is spec'd on a MTB.
Hope this helps

Maybe it will work. It is outside of specs so there may or may not be an issue that would prevent it's use.
Short stay Tri bikes already push outside of specs, so you won't know until you give it a try.

Just buy two long cage and sell the short.
The long cage will run the different clusters without need to change the chain length.

Honestly it's not worth the effort to be swapping them out. I originally considered doing something similar when I built up my road bike a few years ago. It's first trip was going to be to the Italian Dolomites so the mid length cage with 11-32 was a necessity. That was back in 2014 and even though I live in pancake flat The Woodlands, TX I've kept it on there with an 11-32.

Thanks for the advice team. I currently have a road bike I just upgraded to force22 (with short cage) and a tri bike with ultegra short cage. The ultegra is factory new so I was thinking that would have the best chance to sell the take off components on the forum if I replace it with sram so that my two bikes can share chains and cassettes. Just trying to keep logistics and costs down by running the same brand components for both.

Otherwise I'll have to buy two new derailliuers, two new wide range cassettes, etc.

The comment about ultegra short cage running up to 11-32 is interesting and will warrant some investigation.

Changing the derailleur is simple. Nothing complicated involved. You'll just have to set the limit screws and adjust the cable tension. Won't take too long.

If you don't change the sprocket size you may not need to change chain length. If you go bigger you'll need to add links unless the chain was oversized to start with.
When you're on big chainring and big sprocket the derailleur cage is nearly in line with the chain so there probably isn't much impact on required chain length, so I wouldn't be surprised if he's correct about not needing to change the chain length simply to account for the derailleur.

It's not necessary to change the chain length when putting a smaller range cassette on. The critical combinations are (1)big/big, (2)small/small and (3)the difference between the total number of teeth in each. This dictates if the chain is (1) too short, (2) too long, or (3) the derailleur has insufficient capacity.
If everything is okay with the 32 sprocket then you'll still be okay if you change to a cassette that tops out at 28. after all, that's probably the same as the second smallest gear on the 32 cassette anyway.

You would have to break the chain anyhow to swap rd's so it's easy enough to check length and add a link if necessary. But I'm with others that this is too much trouble. Either go directly to mid cage which will cover everything, or suck and see with your existing.

SRAM derailleurs are not compatible with Shimano shifters.

I am aware of that, I would have replaced the TT shifters and FD on the tri bike with all SRAM.

I know that in previous generations, SRAM and Shimano cassettes and chains could be mixed with different derailleurs, I don't know if that is still the case with the 11 speed versions. If so that would mean I wouldn't have to buy 11-28 and 11-32 versions in both SRAM and Shimano.

Just buy one of these and use the same deraileur.

https://www.wolftoothcomponents.com/products/roadlink

The problem is a total capacity one, not cog clearance.