What is good amateur practice if propagation changes during a contact creating interference from other stations using the frequency?
Why No amateur station has a priority claim on any frequency (see 97.101(b): all amateurs must cooperate in selecting and using frequencies). When changing propagation, such as a gray-line or band opening, suddenly puts two QSOs on top of each other, neither group did anything wrong, so the expected response is courtesy and negotiation: agree on who moves, shift up or down a few kHz, or share the frequency. The pool consistently rewards the cooperative, good-faith solution.
Watch out Claiming priority is wrong because frequencies are not owned; the only exception in Part 97 is that no station may willfully interfere, and distress traffic takes precedence. Simply reducing power keeps the mutual interference and just makes you harder to copy.
Nobody owns a frequency. When two QSOs collide, negotiate, don't declare priority.