While I could go either way on the topic so long as there are no mechanical restrictions other than those already imposed by might, I do take issue with this:

[quote]I mean, with the Lich example, the Lich's soul would end up being a completely different one, or an amalgamation of sorts.[/quote]

Don't animals eat physical things and absorb it into themselves for energy? Couldn't liches and souls work in a similar way? I wouldn't say that you're just an amalgamation of what you've eaten in the past month except on a very microscopic level, which isn't exactly practical