The
Stellaris wiki says [emphasis added]:
[The Stellaris wiki is failing to bring up history so I can't link to a ~1.7 version or see if it's changed since then.]
So unless this was a change added after 1.7 you can make whatever changes you want, including a brand new Corvette design, and if you save it as "Table-Class" then that's what your existing "Table-Class" Corvettes will upgrade/change into. The work-around to do what you want (it's a work around since without any kind of UI or anything you're relying on an implementation detail) is to delete or rename your existing "Table-Class" Corvette design (so there is no design with the same name) and then re-save/create your "Bed-Class" Corvette design before commanding half your Corvettes to upgrade (split the fleet manuall first). Then, once that's complete, you'd re-save/create your "Chair-Class" Corvette design before commanding the last of your "Table-Class" Corvettes to upgrade. If this sounds like a fidgety/poorly-designed/pain-in-the-ass hack I'd agree.
Note: I haven't actually tried this so it may rely on an implementation detail post-1.7 or PC specific. Frankly it's such a horrible way to do it that I would never bother.
On the plus side, if this silly mucking around works then you'll have two separately named designs that you can then change at will and as long as you don't want/need to move/upgrade any more existing ships between those deign types then you can just use the upgrade fleet command in the future and your "Table-Class" Corvettes will become the new "Table-Class" and your "Bed-Class" Corvettes will become the "Bed-Class".