I've never tried Welbutrin XL, but your line of thinking makes sense.
When you took the original, you were basically being hit with the full strength of the product right up fron. With the extended release, it is going to put a little but into your system over the course of the entire day.
My reco is that you call your doctor.
Per Drug Digest, it doesn't look like there are any negative interactions when you combine the two:
Check InteractionsNo interactions were found for the drugs you selected.
You searched for interactions between the following drugs:
Lexapro
Wellbutrin XL REF:
http://www.drugdigest.org/DD/Interaction/C...1,4109,,00.htmlSL