I've done a similar task recently - where we over the course of 2 weeks replaced APs.
What worked for me is this:
- add new 7210 controller IP to aruba-master IP
- reprovision all AP125 on 3400 controller - pointing them to IP-Address of 3400 controller
- configure all SSIDs on 7210 controller and connect new APs to this one using DNS lookup.
the controllers will pick up each others APs as rogues and probably your RF would be a bit messy for the time.
What time span do you look at for replacing APs?
it is possible to transfer licenses to new 7210 - and they would be available on 3400 controller also, but not sure for how long. And is isn't allowed. Perhaps your Aruba partner can provide you with TRIAL licenses for 30 days for old controller?
Also - look for trade-in prices for ol AP125s