Updating a very old Asterisk Server - need some advice

I inherited an aging Asterisk 1.2 compiled on gentoo running on a, now ancient, Dell PE 1950. I have a cold spare server with extra parts lying around, but I’m now working on upgrading to the latest versions of Asterisk LTS 13.

I’m rebuilding our, admittedly huge, dialplan from scratch, mapping out everything and streamlining our dialplan using subroutines, and other newer features introduced in later versions.

My questions come down to the following

I’m having issues getting some existing newer phones that have not been used to connect to Asterisk 13. They keep connecting to the old 1.2 Asterisk. I have manually configured TCP/IP and server settings to point to the new server, as well as editing config files (that the phone states it’s using on the LCD screen) on the FTP server to point and login to the new server. Neither works. We’re using DHCP Option 66 to configure the phones, but with static IP that shouldn’t be taking effect should it?

I’ve read elsewhere that it’s better to just point the Option 66 to the new server, but that would break functionality on our existing live setup. Is there an easy way to configure a hardphone to the new Asterisk server?

Is it worthwhile, or even possible, to have the two servers co-exist and migrate slowly to the new system?

I was planning on doing a full cuttover during a weekend, and have it be transparent to end users (as much as possible at least).

What would any of your recommendations be in my situation? Thanks for the help