Here's what I would do:

Setup your wireless IP network between the buildings, create your VLANs etc. and make sure everything is solid.

Replace your STS and get all your KSUs talking to one another over the IP network. Give this some time where you have data traffic and intercom calls over the wireless network, make sure you don't have any issues with the voice quality.

Now you can start moving lines around and have all incoming calls directed at one person if that's your goal. Keep one or two POTS lines on each system though.

If you do want to move to a full IP solution, you can do it gradually. Introduce your IP system and start by adding new employees, and the few people you have that are excited about new technology. In any company you have those people who hate any kind of change, and when you plunk an IP set at their desk, they're going to be a thorn in your side. Leave them till last, and then you can point out all the other people who have already changed with no complaints.