Hello, @Paul Nesbitt !
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others", I'll repost your solution in case you'd like to "Accept" the answer to increase visibility.
We have some VM's in DevTest Labs, which are accessible via PIP. We want to remove that, make them only accessible via our ExpressRoute and join them to the domain. We also want to change their IP's, so need to change them to a new subnet. I've tried doing this already, but it's not possible unless in the same address space, which isn't an option for us.
Solution provided by Paul Nesbitt:
Managed to find a solution. The DevTest Lab is in Southeast Asia, so as we ideally want them in UK South, I used Azure Site Recovery to replicate the VM to UK South. This allows us to move it out of DevTest Labs along with being able to specify a new address space. Once it's failed over, disabled ASR leaving the UK South VM intact and able to be used as a normal VM, now connected to our domain.