I totally second therealgps’ suggestion and share his discomfort. Duplicating racks feels rather unpractical to me and occupies more RAM. So having 16 racks just for the 16 Hammond presets I use in my gig is crazy.
Presets of the same instrument may differ quite a lot, and makes it very unpractical to manually map each of their changing parameters to a different widget so that it can be mapped to a new variation.
Also, I definitely don’t want to use program changes (as suggested as a workaround) to change presets of a plugin, and many newer plugins don’t allow that anyway.
A rack for me would better be considered as a set of instruments, fx and routings that I can modify to different variations (that’s what a rack is in the real world anyway). For instance:
variation 1: ON Hammond preset 1 / OFF rhodes
variation 2: ON Hammond preset 2 / OFF rhodes
variation 3: OFF Hammond / ON rhodes preset 1
etc
So, as a better workaround, I suggest that GP should provide a sort of meta-widget that maps to all the available parameters in an assigned plugin. This may also give the option to ignore some of the plugin’s parameters via checkboxes. This was something available in Brainspawn Forte and it was great!