Jason,
Check the CA DMV website for information on using original old plate(s). Each state has different rules.
In NY, an old plate may be used as long as the number is not being used by someone else. You pay a higher registration fee. You must use the same number of plates used in the year of the plate that you want to use. For example, I have a 1953 NY SUB (Commercial, Suburban) plate and a 1954 year tag (no full 1954 plate was used in NY, it seems). Only one tag is needed because only one tag was used in NY in 1954. NY allows you to use this type of plate (Vintage plate) on a vehicle registered as an antique/collector vehicle or registered as a regular vehicle (no usage restrictions). The plate is supposed to be original (not restored).
Maybe someone with experience in CA will answer with useful information.
Tim
1954 New York license plate
