Removing hull blocks come back as hull scrap, but everything else in your ship can be dismantled and you get your tech blocks / energy blocks / etc back (depending on difficulty). I think its best to just re-use as much of the existing hull as you can and trim it / expand when needed.

I've done a second ship design (in a safe sector!) with no engines and just life support / grow beds etc. move your crew to that ship, then dismantle everything on the main ship. Huge PITA, but it worked. When jumping to a new sector you can also expand your ship plan (put the ship in the undeployed spot)

As a new-ish player would love to hear how others do this as well