Interesting OP, as an exile from Rugby league land this is something I often think about but it is never really popular when discussed by either set of fans - next to zero chance it would happen. However, if it did:
I think the rule set you (Vieux Talonneur) mention is the best compromise between the 2 codes. I suspect it is the locks that would be slowly phased out - I have always thought of RL props and second rowers as basically RU 6s and 8s, loose forwards as 7s. Hookers would be fine. Props, well, I think the more mobile players like Genge and Rodd would do really well, the really big lads would probably be able to play as props but lack the mobility for the new code at top level.
How to change it - well, introduce the new rule set in junior rugby of both codes and let the age groups move gradually upwards so both codes slowly transform.
Given the disastrous financial state of both codes in England it could make a lot of sense here to make a bigger more popular game with a much better TV deal. After all RL is RL only because they wanted to go semi pro and the RU wouldn't let them.
It would have to happen worldwide for it to work. If Australia and New Zealand merged they would most likely become World 1 and 2 for a long time until everyone else sorted themselves out. France would be OK. SA could adapt, they have athletes in their team.