Is it possible to have a site-to-site VPN with both sites on same subnet (temporary configuration while moving to new building)

I have a customer that is in the process of moving to a new building 1km down the road. They would like to keep the network up and usable while they move things one piece/department at a time. They have Windows servers with the PCs joined to Active Directory and file shares.

They have a site-to-site ZyXEL VPN set up between the two locations, but to accomplish that, they created a new subnet for the new location. This is going to be a serious issue when they move the servers. Either the servers will need to be re-addressed making the move 10x more complicated, or we'll need to find a way to keep both sites on the original subnet across the VPN. ​

Is this possible? I found some articles about this, but they seemed to be related to point-to-site (mobile) VPNs rather than site-to-site.

The other option I thought of is a PTP wireless bridge, but there's a few trees and buildings nearby that might make this too expensive.