You can work with old projects as long as they have the cache library intact. Worst that would then happen is that kicad needs to rescue your schematic if the libraries used by it no longer exist in their original form.
An alternative of course would be to have a double library setup where you switch to the old libs while working on old projects. (One kind of easy way to achieve this is to add the old libs as local libraries to the old projects that need them. If it is a version 4 generated project then read my FAQ article about remapping as it gives all the options you have available: Converting KiCad version 4 projects to version 5 (Remap a project))