If I change a project's rate later, do my old time entries change value too?

No. Kliently snapshots the rate at the moment each entry is created, so raising or lowering a rate only affects time logged from that point forward. Everything you already tracked keeps the value it had when you tracked it.

This matters for trust and for clean books: a rate increase in June can't quietly rewrite what you billed in March. If you ever need a different value on an old entry, edit that entry directly — the change is explicit, never a silent side effect of touching the rate elsewhere.