There's an easy answer in most cases: pre-announce the coming price increase and say "Anyone who signs up before then gets grandfathered in at that level for life." Boosts signups temporarily, avoids any blowback to customers who supported you early on, and has virtually no long-term impact to your pricing structure since, to a first approximation, as time approaches infinity all revenue for a SaaS company comes from people who are not currently customers.