The big question is parity at time of conversion. Lifetime US miles are more restricted than lifetime AA miles.
There is a precedent in the CO-UA merger.
CO had an expansive definition of lifetime miles, which was basically on EQMs, including class of service bonuses, DEQM promotions, credit for travel on partner airlines, etc., while UA's definition was essentially BIS miles on UA metal only.
Post merger, the historic UA lifetime miles were uplifted on a one-time basis to match CO's definition, the miles consolidated, and then the combined program switched to the lifetime BIS basis. For historic UA flyers it represented a one-time bump-up so that their more-restricted miles are now counted equally to the historic CO flyers. And thereafter, everyone is on equal footing.
There were some major quibbles with losing some of the perks previously part of the UA lifetime program (lifetime Club access) but on the other hand, new perks were introduced (matching status for a spouse).
The best thing coming out of the combined program - at the 4MM level, lifetime GS status.
I hope they combine the miles using AA's much more generous formula than US's. And I hope they also provide more meaningful lifetime status benefits, say EXP at 3MM and Concierge Key at 4MM.