That's a lot of work for not much gain, without having the original sub-circuits divided out and balanced on the 50A. If you are planning to add several new circuits it might be worth it, but if you are just adding AC, I personally would upgrade to the 50A cord but just have it feed your existing 30A service on one leg, and a new separate 20A breaker for your 2nd AC on the other leg. You would still have the advantage of 1 cord to plug in, but a lot less work inside.
PS - in 2003, that was how my factory Montana was wired with equipped with the 2nd AC rough in. The master breaker panel was simply a 30A and a 20A breaker. Not near what the true 50A service is, but it allowed for a 2nd AC and 30A service using one cord.