Yes, the CAL is completely convertable back to standard config, however it will require some metal work that most of us cannot do and those who can will charge you a lot. The standard M82 holds its magazine much like an M-14: It holds the magazine in both the front and back. For the CAL version, the mag well has been milled away at the front to acommodate the hinge arm of the magazine. The front magazine load bearing point is now gone, it has been moved back to the hinge just behind the bipod mounting bracket. Modifying it back to standard M82A1 config would require extensive welding and milling in order to put the front magazine mount back where it should be to handle a detachable magazine. This is going to be very expensive. It will also require refinishing of the entire receiver. Remember, the CAL version was approved by CA DOJ. It cannot have ability to accept a detachable magazine, so there is no mechanical provision to accept one. You'd have to rebuild that ability.
You'll looking at $500-$750 just to get it back to M82A1 config, money that you will not get back from a sale. No one will pay M82A1 prices, even used, for a modified CAL. If you love the gun, good on ya. But if its resaleable value has any impact on your decision, I'd think twice about it. It'll cost you dearly.
There's also the longevity of the CAL version. With that swing-down magazine, reloads are going to prove awkward and I'll put money on someone dropping the gun during this awkward and vulnerable reload period. When 30 pounds of steels lands squarely on the magazine, it will destroy it. Give it a year or two for this to happen, long enough for Barrett to get very busy with other things and forget about the "favor" they did for Californians, then call Barrett looking for that replacement part. They'll either not have it (after all there were only 100 made) or they will charge you a fortune to repair yours.