I think that I would switch my host if I felt that I wasn't getting the same quality of both services and support that I had been enjoying.
This is often the case when a company changes hands. I've noticed this with Forumer's IPB forums. They ineffectively managed their IPB forums and when their company was purchased, all of the IPB forums became Yuku forums.
I think that if my host owner ever let quality suffer to that extent, then I would most definitely change providers.
I'm on a free host now, and although I've discussed moving to paid hosting with my members and staff, I havent really committed to doing anything as of yet as I don't feel like I am missing out on anything by sticking with my current host. My members often feel the same way and newer members are surprised that it isn't paid considering the level of freedom we have with mature content and the fact that I have free access to my forum's database and can back up my forum at any time and take my exported SQL file to any host and convert it to any software I choose..