If you are talking about the "state" registration numbers, they are usually "stick-on" and can be removed by heating with a heat gun or perhaps with a hair dryer. You may be able to minimize the marks left by the old numbers with rubbing compound and then apply a good polish/wax. If you are talking about the actual hull numbers that are assigned by Catalina, I would not remove them.
If you know anyone who works at a hospital or Dr's office. The adhesive remover wipes they have are the bomb for getting leftover adhesive off. They work much better than WD 40. It also helps to have one of those plastic tools that body shops use to remove trim without scrating paint.