8.3.3 to firmware later than 8.5.2.

Warning: you SHOULD NOT use this plugin if your 7900 phones are using SCCP firmware version earlier than 8.3.3.

It's not possible to upgrade a phone with firmware earlier than 8.3.3 to firmware later than 8.5.2. The phone will usually reboot in loop in such situation. You should upgrade your phone to firmware 8.5.2 before upgrading to firmware later than 8.5.2.