So is this the answer to whether or not we need to sort out a JAR-PPL 'upgrade' for our old lifetime CAA-PPLs?
I would not do this unless you have a pressing reason to e.g. you want to go to some JAA State, which does not accept the CAA PPL, and rent stuff there.
The counter argument is that it may cost you less to do the swap now than say next year, but the difference is not likely to be significant on the scale of flying costs - even if a checkride is required. If EASA terminated national PPLs in some crude manner, a lot of **** would hit the fan, and of course they cannot do anything to the French.
In this business, people who took drastic action in fear of something happening usually got shafted. For example, those who got scared of the 2005 DfT proposal to end N-reg long term parking and spent £thousands to go back on G-reg were soon p*ssed off because they wasted their money, as well as depriving themselves of IFR privileges of their FAA IRs.