No, this is not a really old sales pitch. I'm not trying to get you to buy a LCB, I'm telling you to sell it. Then you can buy a new one. Rinse and repeat.
- Mr. Dockmaster! Are you sure there's no more of those 35Cr/t Computers left?
- Yes, we only had 735 tons, and you've bought them all.
- All right, give me 500 tons of that 20Cr Machinery instead. And a ton of Liquor/Wines for, uh, representation.
- Yes, sir! Where would you like it delivered?
- Oh, just load it in that old Cobra over there
-

But seriously. (And I apologize if this is old news - I searched the forums but found nothing relating to this.)
Running sell_equipment 1.3 with v1.73.4 under Windows XP, selling the Large Cargo Bay removes the item from my equipment list, but does not reduce the cargo capacity. Buying a new LCB adds another 15t capacity. And another 15. And another. And...
Tested with a clean install running no other OXPs. The excess capacity seems to be fully usable - I can buy everything the station has, and still have room to spare.
EDIT
As per Micha's suggestion, I tried save/reload. Extra capacity gone. So it's more akin to buying a big, one-use sled to tow your cargo on then.

Probably should have spotted that before posting, but it was 5 am, and waaay past my bed time.
/EDIT