The 340 has the same seats as the 330, the only difference being two more engines and a different seating config on the 340.
You are in Affaires. First, it is impossible to book La Premiere with skymiles and also it is not even offered on the 330/340.
Correct. OP, I wouldn't worry about the switch, it shouldn't make a difference in your flight quality. I have only flown AF J once and liked it very much (but it was my first J flight, so anything above Y was bound to be amazing).
FYI, unless I am mistaken, AF does not operate the A340-500, and never has.