@WhatDoYouMean: Well, like blowdart said up above, you'll rewrite it all anyway if you're leaving the platform entirely (to PHP?).
But, if you wrote code in tiers, and had a business layer + data layer containing all of your business objects (reference them in your models), treating the MVC application as just a client app that consumes your business objects, then you get the flexibility to "move" when you need to. Just construct a new "web forms" application that consumes the layers you had before.
