I'd leave it as Is , just change the Chainrings so the Gears I Like are In the most convenient relationship to the shifting pattern ..

Ie Not needing a double shift , of both derailleurs at the same time to go to next gear in sequence .

speeds is just a cog count of how many are packed in the space.

to go from 7 a 126 wide frame to 8 took spreading the rear stays.. as the 8th cog made the cluster wider..

at 8 the freewheel hub axles tend to bend more, then a cassette free hub wheel will be more durable.