He clearly still believes in police reform. In some places that includes reducing police budgets in favor of more effective public safety programs. That's what "defund the police" means. Not "abolish", reduce funding. In NYC he's running on maintaining police funding at current levels and adding additional nonviolent peacekeeping capacity. He may personally believe that ultimately funding can be redirected further, but that's not what he's running on.
Criminal justice is majorly fucked in the US broadly. We incarcerate non-dangerous people with minor offenses way too long, and we let dangerous repeat offenders walk free. The answer isn't so simple as "lock more people up" or "let everyone go", we're in a trickier bind than a straightforward over incarceration or over lenient set of policies. Mamdani talks a lot about reducing penalties for minor nonviolent offenders, and for increasing rehabilitative capacity, but he does retreat to rehabilitation too readily (from a rhetorical efficacy perspective) when questioned about how to handle repeat offenders.
I don't think he's actually changed his values at all, he's just polished his phrasing and set more achievable near-term goals.