You may want to check out batlabs to get the skinny on flashing.
Motorola Flash Programming Primer
Typically, if your not the original owner of the radio (meaning you bought it new from a dealer) Motorola doesn't like to sell flash upgrades. If you are the original owner, your fine, just talk to your local dealer about the upgrade. (It will cost money tho)
On the ASTRO 25 series of radios, which is yours, I believe a firmware upgrade might do it. (I could be wrong) Some people may correct me if I'm wrong, but I know you can "dirty flash" the radio, which will upgrade the firmware. If anyone knows if that actually adds "IMBE digital operation" let me know! (Otherwise that may have to be flashed supplement to the firmware upgrade) I have a friend who took an MT 1500 and "dirty flashed" it to add P25, so I do know it can be done.
Also, for quick reference, any flashcode starting in "5" or "1" is capable of digital (P25) operation
Cheers!