If you apply rules and then simplify you may get something like.
(25 A^2 hbar Sqrt[π])/Sqrt[hbar m w]
How would you get mathematica to rewrite this in terms of E where
E -> Sqrt[ m w/ hbar ] x
It's very easy to plug E into things, but I don't know how to get it back out. Further, I keep running into situations where I wish I could do this, and then end up having to finish what I am doing on paper. Any ideas?