Currently, you can order awards on AF using FD miles in eco and biz (not in first class). The applicable 'tariff' is that of FD.
You also earn FD miles on AF and, again, the applicable number of miles is the FD one. Both are mileage based, but you get differences on minimum miles, CoS and Elite bonuses.
AFAIK, you will not get extra FD miles for flights flown in first (i.e. these are treated the same as business class).

One-way, one-class upgrades on F+ are half the miles of a return economy class award (in other words, they cost the same as an economy class award ticket)
but
you can officially only upgrade from a full fare ticket (in practice AF K class is also apparently allowed) and on AF metal only. So a very poor deal compared to FD.

So, if you want to use your FD miles to upgrade, order your upgrades this year, while FD still exists.

If you intend to use your miles for a non-upgrade award ticket next year, it would probably make sense to wait for the merger if you can, as award tickets are likely to be cheaper.

I would expect the conversion to be 1 FD mile = 1 F+ mile = 1 whatever the new programme currency will be.