The cases would still be evaluated as per the state of the law as it stood at the time the questionable acts occurred. If police were operating under QI and QI is later repealed, changing the law's interpretation of their behavior after-the-fact to find them liable is ex post facto law.

Sucks and is unfair, but it's the same principle that stops the government from, for example, jailing people who used to drink if Prohibition were put back in the Constitution.(1)

(1) There's a carve-out in jurisprudence that if something was illegal in the past, and you're jailed for it, and it is later made legal, your continued incarceration can be re-considered. But that's handled on, generally, a case-by-case basis.