The only difference between a satin clear coat and a gloss clear coat is the amount of polishing that happened when the made the bass! It's sprayed, then buffed, then sprayed, then buffed many times. When the finish has been built to the proper level, it's polished using big buffing wheels with progressively finer abrasives. If you go to the finest polish, you'll have a gloss instrument. If you stop before the last polishing step, it will be satin.
So, if you really want to go from satin->gloss you just need to find somebody with the equipment to do that buffing. Any woodworking shop can do this sort of work, but they'll need your instrument to be completely stripped to do this - all hardware and electronics need to be pulled off. Since we're not talking about a Precision bass here, this means you really should probably think about sending it back to Alembic so something bad doesn't happen in the reassembly.
I believe that Alembic may be using a slightly different varnish for gloss instruments. Back in the old days (the 80's) everything was finished in polyurethane and most instruments were satin. These days, I believe Alembic said they use polyester finish on the high end instruments as the hardness (it must finish harder than polyurethane) lent it self better to a mirror finish. But you should be able to achieve a gloss finish with any varnish unless it's been blended specifically not to be glossy.
It's the same varnish, so it shouldn't affect the sound, but may affect the playability - you may find that the gloss finish will have more friction than satin.
