Most Sample Based VSTi allow you to pre-load voices (Including drum kits) into a Multi, (Usually a bank of 128) therefore it is quite possible to switch between voices with no delay whatsoever, (This is true for styles and Midi files and not just voices) unfortunately there are a few were you can�t, so for these you will need to load 2 or more instances of the program into your VST Host (Having a different voice in each instance) and switch between the different program instances. (There will then be no delay when changing voices mid song)

Hope this helps

Bill