from technical view:
now the AP should join to XCA.
This is the current way.
For flashing AP39XX to Wing 7.4 I can’t give you information. AFAIK it’s not available yet.
But when it is available, the XCA should have a image or process for flashing these APs. All other will be a nightmare.
Extreme Sales are able start a migration process for AP-CAPWAP Licenses.
Limitations? This depends, what you’ve configured in identifi.