Best practice -- and I'm shamelessly stealing this lesson from Joel Spolsky -- is to grandfather SaaS accounts in indefinitely if you raise prices. This a) lets you get a nice bump in sales if you announce the incoming price increase (no reason you can't do the A/B test quietly but announce the conclusion that prices are going up "loudly"), b) rewards people for taking the risk on you "early", and c) incentivizes them to keep their account active because if it lapses then they lose their super-sweet not-available-anywhere deal.
If I were to redo a pricing scheme in such a way that some customers would benefit from switching, I'd switch them automatically if the new pricing dominated their existing plan ("More quota for less price!") and tell them to make the call if it weren't obvious ("More quota for less price but this new plan doesn't have a particular feature that you may want to have in the future.")