I have a set of the FLs on my Fender P. A couple weeks ago I went from the 45-95 GHS precision flats to the FLs. It was a bit of an increase in tension which took some getting used to, but now they just feel awesome. I did not have to adjust the truss rod.
Within reason, as long as it's not a major increase in tension, you will adjust to different tensions in a short period of time. I can't imagine there's a huge difference between the FL and FS sets. If it's a concern, start with the FL set.