Well, each individual is complex in their own right -- experiences construct our identities, alongside our self-perception and how we socially engage with each other. Therefore our contemporary personalities are forged by multiple forces interacting simutaneously. That being said, and keeping Mr. Merlotte's privacy in mind, I find it unlikely that his stability would regress quickly... Unless someone from his personal world, to include his recent past, imPorted into this one.

Which is, we all must admit, not entirely impossible.