Can an assembled 19-inch cabinet be dismantled and reassembled later?

Yes.

The 19-inch cabinets use essentially the same bolted construction whether they were originally supplied flat-packed or ready assembled, so they can be dismantled and rebuilt later if necessary.

That can be useful when moving a cabinet through a restricted opening or relocating an installation.

Before reducing the entire cabinet back to its component ancestry, though, removing the quick-release door and side panels may already reduce the size and weight enough.

Miniline cabinets are different and aren’t intended to be dismantled in the same way.