The kicker here is that MetroPCS could migrate off of CDMA pretty quickly if it needed to (e.g. when merging with TMo). The company already has one VoLTE-capable phone and they're pushing hard for more. Of course it might be cheaper to just give everyone on MetroPCS an HSPA capable phone comparable to what they have now on MetroPCS, compared to getting VoLTE running on TMo's entire network on a quick schedule. But it's an option.
As for spectrum, TMo and MetroPCS fit perfectly: PCS and AWS. The key for T-Mobile will be shutting down CDMA on AWS quickly so they can use all of MetroPCS's spectrum there for LTE.
TMO seems to be pretty good at refarming the spectrum. I don't think it would take them long as long as they'd set a plan and make it soild and just roll with it. Even if it means moving those customers over to TMO handsets. TMO could give out free phones.