Is it possible to replace an iMac Fusion Drive HDD with a bootcamp partition on it with an SSD using migration instead of reinstalling Windows?

I've never done it in this particular circumstance, but armed with somewhere to mount the new drive while you do this [& belt & braces, a regular Time Machine backup], WinClone & Carbon Copy Cloner, I'd say you can't go wrong. Each of those apps will help you through each part of the process, making sure you get a Recovery Partition etc.
At all times you retain the original drive in its original condition, so you're pretty safe. Swap them over once you're totally happy.

It's the kind of task I've done many times over the years, but I have my Boot Camp on a separate drive entirely, so I don't need to move it around too frequently.