Yes it is of course possible to upgrade your group of AP1221s to the last Stellar AWOS. Effectively you will need to upgrade with last 3.0.6 to manage a group of 255 APs.
Note also in Express mode you can only upgrade by AP model. click on the AP-Group menu and "upgrade all firmware", then you can expand each model to upgrade with its related image file (one image file is available per AP).
Pls note upgrade is done one shot per model in a group and takes 5 minutes minimum, you have to find a relevant downtime to do it! For a group of more 50APs I would advice to do proceed by steps, first with selected set of AP12XXs, check your upgrades then terminate and the end with AP1101s if you have.
I would recommand at a moment to envisage connect APs to the CLoud (you have the button) where you will have possibility to upgrade with a refined upgrade (per selected APs) and the possibility to program your downtime.