The obvious answer is to check the directions, and with the manufacturer, of the new paint. If the paints are compatible there is no need to remove old paint that is adhering well, although it should be sanded for good adhesion of the new paint. Failing all else, you can check the compatability of new and old paint by soaking a rag in the specific solvent recommended for the new paint and let it rest on the old paint for ten minutes. If the old paint comes up you will need to use a primer to seal the old paint first. The problem arises from the solvents in the new paint which will reactivate the old paint prior to flashing off. Ignoring this can cause a hopeless streaking of new and old paint as well as adhesion problems. It does not matter how long the old paint has been on for this to occur. Again, your supplier if knowledgeable, and certainly the manufacturer of the new paint should be able to advise a course of action and recommend an appropriate primer as necessary.