Safe Harbor Mid-Year Change

Can a safe harbor plan change to immediate eligibility and vesting mid-year? I believe they can with notice, since it's increasing benefits. But does that then mean anyone not previously vested would become 100% vested?

Yes, with proper notice as you indicate.   All participants with an hour of service on or after the effective date will be 100% vested, For this purpose the effective date can be earlier than the adoption date.