Wireless Access

Currently have 2 controllers (master/standby- running vrrp)..with about 40 AP125's.  Looking to migrate to 40 IAP's in 3 stages (about 13 AP's at a time).  Can both environments be up and running in parallel for a few months?  Want the 1st stage of 13 IAP's pointing to the instant cluster VIP while the existing AP125's still pointing to the physical controller.  We will be using the same security and encryption and want to stay with the same exact SSID's.  Will this work?

Technically it will work but I would try to do a floor/area at a time to avoid clients roaming from controller to IAP. Roaming between these two environments will create a re-auth attempt and not be as seamless so structuring the physical separation would be the highest priority. 

If that cannot be done, I would disable ClientMatch if you have a controller code that supports it so that ClientMatch isn't causing aggressiveness in trying to move clients in which case an IAP is acceptable in this area. 

Also, be careful that any rogue containment isn't configured to avoid knocking off clients on the IAPs

Check to see how tolerant your clients are to the re-auth. We're running fairly sensitive clients, but nothing requiring real-time-protocols. 

I was able to make that transition without bothering 98% of my users.

We were able to show lost connections and missed pings during roaming, but the clients almost never noticed.

Your mileage may vary of course.