I have an MVP410-FX, is it possible to change this device from SIP or SIP to H.323?

The MVP410-FX was designed as a SIP and SPP gateway - there is no firmware that will allow it to support H.323.

The standard MVP410's support SIP, SPP and H.323. You can still make a call from the MVP410-FX to the MVP410, but you must use either SIP or SPP protocol.