I did the switch. Didn't notice much difference in effect -- neither worked for me very well -- but the side effects were a bit different. If anything, contrary to what I had heard, the Celexa has had fewer side effects. So they don't feel the same, but my guess is, if Lex works for you, Celexa probably will as well but it's not a guarantee, they do feel different.
They are very similar medications, although there are some important differences between the two.
Celexa was available first and is a mixture of R and S enantiomers of citalopram. Lexapro is just the S enantiomer of citalopram.
Depression research has shown that the S enantiomer of citalopram is much more active as an antidepressant than the R enantiomer, which is why Lexapro (just the S enantiomer) was developed.