Is there any way to move an NPC from one house to another in Terraria?
You would need to
Destroy his old house and build the other one (he will go to the new house when it becomes night if the distance isn't to far)
Destroy his old house and build the other one and hope that the same type (merchant, gun dealer etc.) will respawn
We once had 2 merchants; when we tried to get a Demolitionist, we killed 1 merchant and got another 1; after we killed that one, we finally got the Demolitionist.
P.S. You only need to hammer away the chair and table to destroy his house