Hello Matt,
To download the firmware I believe the APs should be under a support agreement. The conversion version was 5.9.2.0 but of course we recommend getting them up to the latest 5.x firmware and if expansion into the 11ax universe is planned then to 7.x firmware.
Here's a link on how you may download firmware: https://extremeportal.force.com/ExtrArticleDetail?an=000085387
I hope this helps,
Chris
Christoph S.