Interesting question. Allow me to rephrase...
The older airplanes will go to GA TRK, and once you push lateral on FCU, engage NAV. The newer aircraft (or modified accordingly) will go to NAV straight away.
Now, once in NAV, will it continue along the FPL trajectory, or jump immediately to first MA waipoint (blue on ND)? Assume the first MA wpt could perhaps be 800+ with a right turn to a heading.
I am inclined to believe, that indeed the lateral guidance will follow the programmed FPL-A sequence all the way to MAPt and only then follow the GA track. However, I am unable to find a definitive answer in the manuals as of now.
Cheers,
FD.