I don't know Bose specifically, but I would guess that the unit is not upgradeable to support newer surround sound formats. Some higher-end units can do this through firmware upgrdades (whether the manufacturer actually ever releases an upgrade is another issue), but as Bose is marketed more towards the consumer market, vs. the "prosumer" market, I would be surprised if they would put such a feature in their products. If I were Bose, I wouldn't be convinced that there were enough people in my target market would understand what a "firmware upgrade" was and be able to apply one to justify the development costs. Not slamming Bose at all--just my frank conjecture.
On another note, THX is not a surround sound format. THX is a specification that sets standards for various theater-related issues. For example, a THX-certified movie theater can only have a certain maximum horizontal viewing angle to the screen from the furthest-side seats, a certain maximum vertical viewing angle to the screen from the closest seats, reverb must be below a certain point, etc... THX certified stereo equipment is designed to meet certain minimum power, signal-to-noise, etc... standards. It is also sonically designed so that THX-certified equipment from different vendors is supposed to be sonically "compatible". Either your Bose is THX-certified or it isn't, but a firmware upgrade isn't going to take it from one to the other.