I did that move many times in my 8 years in the enterprise field(I hold MCP, MCAD, MCTS), I won't be surprised if you have to work with more than just an old version of .NET. The PC market is full of legacy stuff, you go back to work with those .NET2.0-ish people and you end up by dealing with VBScript, COM+, ASP, VB6 and stuff like that. I personally think that you will not be entirely happy in an enterprise environment, so I would work with other more interesting technologies in my free time.
Also, for personal experience, take in count that companies working with old technologies rarely want to sponsor certifications for newer version of the platform(forget about C# 5 free exam vouchers) since their ".NET 2.0" infrastructure is working "just fine".