Yeah, generally you can run two lines through the same block briefly (only needs to be done on the leeward side), then pull out the old one as you douse the old kite. Windward sheet can wait until the old sheets are below deck.
If you can hoist outside, yeah, but outside hoists are usually pretty messy and it may be faster to just set everything up and barehead it. If you peel (first peel) then gybe, the second peel can usually be done as an inside hoist on the upper halyard. You can always hoist inside on a [clear] lower halyard, but an upper halyard needs to be to windward of