I am replacing two 21 year old Carrier AC units with new Carrier 10 seer units, coils and lines. My installer said that he will use the old cases ( as they are better constructed than the new cases) to house the coils. Is this an OK thing to do? Are there any size concerns to address with doing this? Should I require them to replace the cases? Thank you!

I am guess your installer knows more than anybody here. They have seen the installation. The housings are no more than a metal box. If the old ones are in good shape and fit the new coils, I see no problems. The A-coil needs to fit the housing so that air must go through it rather than around it. Putting your money into metal in the coils and not the housings, may be a smart idea.