Rip the band-aid off or grandfather existing SaaS users?

There seem to be two schools of thought when raising SaaS prices: either lock in existing users at their old rate forever to build loyalty or give them a short notice and upgrade everyone across the board. We recently pushed through a long-overdue rate increase with zero marketing fluff just a plain explanation of why support load forced the change. I was terrified of losing our early adopters but the pushback was almost zero.

If you've pulled off a price hike, did you lock in old users long-term or force everyone onto the new plans?