Footprints are included in the PCB file so there is no need to worry regarding reorganization of libraries (the old project will not be affected)
Symbols from version 6 onward will get the same treatment. However all versions of kicad up until version 5 sadly do not include symbols in the schematic files. The only safety measure in place is the so called rescue mechanism which relies on the so called cache library to be valid.
3d models are another thing that is not stored as part of the project. So if you want to preserve them then you need to take care of them as well.
If you now decide that you want to have all library assets archived in the old project then you can use the archive project plugin.
Version control tools like git are of course another option.
Further reading: