Do I have to remove an old sealer, or can it just be patched?

Some Aldon sealers can be patched simply by adding more sealer to the effected portion. These are the petroleum solvent based sealers.

However, generally another companies sealer that is failing is not worth the effort to try to patch it. It is better to just strip it and redo it, because if it is failing in one place it is likely to fail in others.