When it comes time to apply topside paint to your boat, compatibility with the substrate is a very big factor in how you will prep and which paints you can use. In addition, bare fiberglass, epoxy primer, and bare gelcoat will require specific preparation for each paint type.
What paint do I have on my boat? The most relevant thing to figure out is whether it is a 1- or 2-component paint. 2-component paints such as Awlgrip, Awlcraft, Epifanes Polyurethane, Quantum 99, or Interlux Perfection are much harder, and will generally accept a 1- or 2-component paint if the substrate is still in good condition. If the substrate is a 1-component air dry paint, it's a little softer and can not usually tolerate the solvents in 2-component paints.
How can I tell if it's a 1- or 2-component paint? First, 1-component paints will generally have a service life of 2-10 years depending on usage, UV, and environment. Many people have been able to stretch a 2-component paint job for 5-20 years or more. Knowing what brand of paint was used, or how old the paint job is can also sometimes help. Another test method is to take a cotton rag wetted with acetone and wipe an inconspicuous spot to see if it gums up the paint, removes it, or if it does nothing. 2-component paints commonly will not be effected by acetone, while 1-component paints tend to degrade from the solvent.
Gelcoat and bare fiberglass will usually accept 1-part topside paints directly, or using their proprietary 1-part primers. When applying 2-component paints to gelcoat or fiberglass, these paints generally require their proprietary 2-part epoxy primers to form a solvent-safe and inert surface for them to bond. Epoxy primers are usually safe for 1-component paints as well, but are usually required for 2-component paints.
If it is ever a concern, it's always safe to completely remove the previous coating back to fiberglass or gelcoat and prime as directed by the topside paint of your choice. From the paint manufacturer's standpoint, it is always best to do as directed in the instructions on the paint can label.