The policy in force at the time is the one that answers
Personal liability cover generally responds to incidents that occurred while it was in force, even if the claim is made later. Someone injured on a property in 2024 can bring a claim after you have sold it, and the 2024 policy is the one that matters rather than whatever you hold now.
Which means past limits matter permanently
If your limits were low in the years you owned something, that is what stands behind a late claim from those years. Raising limits now protects you going forward and does nothing about the past, which is an argument for carrying sensible limits all along rather than only once there is something to protect.
Keep the paperwork after the sale
Declarations pages from prior years are what establish you were insured and for how much. People discard them on closing, and a late claim is exactly when they are needed.