In general, ablative paints all go over other ablative paints. These paints are designed to slough off, exposing new biocide.
So called hard bottom paints such as VC17 or Pettit Vivid generally require removing old bottom paint and painting fresh.
If you're going to the trouble of stripping, sanding, or media blasting old paint back down to the gelcoat, you may consider applying an epoxy barrier coat to limit water intrusion into the gelcoat, and help prevent osmosis (blisters, pox, etc.) and then paint with your preferred bottom paint. If you do go this route, make sure you use the recommend de-waxer - improperly removed, or not removed, mold release wax stays around on gelcoat for a very long time, and will ruin all your hard work.
The theory of ablative paint is that it will wear down and won't build up. In practice, I think it wears mostly unevenly, and periodically you'd want to strip it down and start over, depending on how fast you want your bottom to be. Or how perfectionistic you are, etc.
Many folks paint an initial, contrasting color of anti-fouling paint, and then finish colors over that. When the initial color starts to show through, then they know it's time to repaint.
Last year, I painted 1 coat of Bottomkote NT over existing ablative, which may or may not have been Interlux based on the PO's recollection. I noted that the waterline was especially blotchy before I painted, and the Bottomkote mostly wore off the waterline similarly. This year, I painted 2 coats of Pettit Hydrocoat SR about 2 roller widths down from the waterline (10" or so,) and 1 coat on the rest of the boat. Application instructions recommend 2 coats, but 1 quart won't cover 2 full coats. I'm hoping the SR will prevent the brown slime that is common in our lake, and any anti-fouling seems good enough to prevent briozome formation (jelly globs.) We'll find out in October!
