TotalBoat Halcyon varnish can be applied over latex paint, as there are no compatibility issues. The latex paint would still need to be abraded with 320-grit sandpaper, then all sanding residue and contaminants need to be removed prior to application.
It should be understood that Halcyon is a much harder and more durable coating than latex paint. If the coating is stressed or flexed over time, it may lead to a premature coating failure. This could range from Halcyon releasing from the latex paint, or the Halcyon pulling the latex paint from the substrate underneath, or a combination.