As additional information…
Traditionally, phones attached to DPMA have their web UIs disabled, to prevent any web config changes made by the user interfering with any server-provided configuration elements. Given that, for phones that are so-attached (DPMA+Asterisk, for example), you can, beginning with firmwares 1_4_1 for the D80 and 2_2_1_7 for other models, browse to http://[ip of phone]/firmware and, after logging in with admin and with the phone’s web UI password, change-out the firmware on a phone-by-phone basis. This is possible unless you’ve got the server enforcing a firmware otherwise - if you’ve selected a firmware for a phone via the phones-addon for FreePBX, and it’s not blank, then you’re enforcing a firmware.