That's definitely a huge move from a php based blog to one coded in ASP.net.  For starters you'll most likely be switching from Linux web hosting to Windows Hosting which can have it's challenges.  I would seek the opinion of another designer and I would also get the exact reason why your developer thinks that's such a great idea.  If you're interested we would definitely assist you in the migration to any blogging platform if you had a dedicated server with ITX Design.  ITX Design provides managed dedicated servers so something like this would be covered under regular support.