Caged balls are a convenience issue, nothing else, but then only if you going to do a replacement and not clean and repack. If you plan to open the assembly, clean everything, inspect the balls with a magnifier and then replace them all, a caged set just adds one step to the process. If you are going to open, chuck and replace, a cage is convenient.
Nowadays caged sets are available only in poor quality balls, class 200 or perhaps even worse.
Retro-fitting cages is only possible if you can source the right cage. It is just now worth it, use what you have.