I did my JAR PPL conversion to FAA PPL, JAR stays allways there unaffected as you where asking. To put it more easy for you I have been back home in Europe flying after that. In your FAA license it will state only valid when accompained with license XXX, that would be your european one..limitations, restrictions apply etc..
The process was very simple
Same day i visited the FAA they handed me over with a temporary license (while the make the original one) and i was able to go fly.
Normaly an Instructor will do a biennial flight review wich requieres min. 1 hour ground 1 hour flight. Just to make sure you now the airspaces, use of radios etc. Mostly because from time to time some one goes in to class B airpace with out clearence or other serious incidents.. Enjoy ur flying!