If LINQ to SQL isn't broken, why try and fix your app to use EF 4.0?
I know it's no longer a namespace that will be updated, but it should continue to work for your apps for a very long time.
We did a new app last year using EF, and there is a learning curve and nuances to overcome.
Looking back, I'm 50/50 on my decision to use EF. There are some advantages, but I'm not sure it's worth it at this time.