So I am looking at zitifying a windows server that is currently running another ZTNA application. Unfortuantely, it is using an IP in the same subnet. So, I can't do a parallel migration, or even test it on a small subset of clients.
Is it possible to specify a different subnet?
Are you saying the other ZTNA app and OpenZiti are fighting for 100.64.x.x address space?
Yes you can do that:
Main menu -> Advanced Settings -> Tunnel Configuration -> Edit Values
Use values: 172.16.0.1 and /16 and it 'should' work (I don't have another ztna implemenation handy to test/verify)
When done your adapter will start at 100.100.0.1
Unknown adapter ziti-tun0:
Connection-specific DNS Suffix . :
IPv4 Address. . . . . . . . . . . : 172.16.0.1
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
It's worth noting that "other ZTNA apps" can (and might) fight and cause unexpected results. So, no guarantee it'll work but it "probably" will... 
Yeah, we are gonna find out, lol.
Thank you very much for that. I will let you know what I find out.
edit: ohhhhh... now I get it. the desktop edge app is what I am changing, and the IP on the identity... not for the whole environment.
So, that means that I should be able to install the identity on the server, and set it to non-conflicting subnet. Then on the client machines I either need to do the same thing, or stop the existing ZTNA app.