[mswen]

I would engage but not assume it will last very long. In fact I would do a three pronged approach early on. 1) I would identify a particularly nasty corner of the code base that could be sectioned off and improved in such a way that everything that touches it will be easier to deal with. This prong is your short-term easy win "we did something to make life better."

2)Develop a reasonably detailed plan for complete update and refactor within .NET environment. This preserves the legacy, skills and domain knowledge of the current staff.

3)Develop a broader plan for a green-field rewrite in a modern environment with great developer tools, training and support.

My guess is that they will not take either of the more radical approaches but you will have delivered consulting on what they can do to speed up their development process. They might not like the implications of the answers and at that point you can probably just walk away.