Hey Miguel, you will not need to drop, change, or mess with your, or your wife's plan. After they get the merge going and metro is officially running on t mo's gsm towers (while still operating their cdma towers) You can get a sim card from metro put it into the nexus 4 and have it run. At first it will just run on tmo's network but after they retrofit all the towers the combined company towers will put out the same thing.
I hope that was clearer.
The only foreseeable problems I can tell will be getting faster LTE here in Florida because tmo has not yet got around to rolling out lte.
However your metro account will still get/use the available metro LTE, T-mo subscribers will not get to use that metro LTE until later in the process.