One WAP server can only use one ADFS farm.
Is your ADFS on Windows Server 2008 R2 a farm deployment or a stand alone deployment?
If that's a farm deployment, you can actually do a parallel run upgrade. It is the same process as in: https://video2.skills-academy.com/en-us/previous-versions/windows/it-pro/windows-server-2012-R2-and-2012/dn486815(v=ws.11)
If that's a stand alone deployment, you will have do an actual migration. In that case you will need to bring a new infra with WAP and ADFS. And if the challenge is that you just have one public IP for the WAP, you could in theory publish the new WAP in a pass-through rule on the old WAP. DNS will have to follow, it will also break certificate based authentication (just in case you are using it).