go to virgin mobile usa website, sign into your account. find "swap phones" enter serial number of triumph and activate it will deactive the OV.
the ov will still work, you can use it like normal but it wont have 3G or calling or texting, basically a tiny tablet.
you can restore OV to stock using the flashable stock rom found on the forums, flash it like u did with cm7 you can do this before or after you swap to the MT. doesnt matter.
on your old OV, if you want to sell it, you HAVE to tell the new owner that when they activate it, they must call VM tech support to get their MSID, so they can enter it into the OVs programming menu. this is needed because it still has YOUR msid, and while its not active as a phone, that MSID will be incompatible with the new owners number and wont work.
the activate app is supposed to clear this, but it doesnt, you need to do it manually in the phone, and need to call VMusa to get the number, becuase they dont just tell you that, like they tell you your phone number.
this only applies to whoever buys or uses your old OV. you should pass along this info so they dont come back mad at you that they cant activate it.
the new MT should activate on its own just fine automatically through virgin mobiles online process.