Grandfather legacy users or move everyone to new pricing?

Tradeoff question about SaaS legacy plans and new pricing structures.

Legacy plans can preserve trust, but they can also create billing and support complexity. For a small product, grandfathering early users may be the right trust move. But if old plans no longer match support cost or feature packaging, the team may need a migration rule. Where do you draw the line between honoring legacy plans and simplifying the product?