Vesting Schedule with Merging Plans

Was hoping anyone has experienced this before and any thoughts. Am trying to think through all possibilities and with compliance testing for the HCE and NCHE employees also rehired participants. The thought is to merge plans so not one is terminated. 

Vesting Schedule

Plan A - For hires on/after 1/1/2017, 25% after 2 years; 50% after 3 years; 75% after 4 years; 100% for 5 or more years, 100% vested if employed on 12/31/2016 

Plan B - 3-year cliff; 6-year graded vesting for pre amendment of 12/31/2016

Possible Vesting but what happens to rehired participants and compliance testing? are all of these possibilities?

1. Apply the legacy vesting schedules to amounts accrued through 12/31/23 and a new schedule to amounts accrued on/after 1/1/2024.
2. Vest all participants immediately 
3. Vest 25% after 1 year, 50% after 2 years, 100% after 3 years 
4. Changing everyone to 100% after 2 years 
5. Vest everyone 25% after 2 years, 100% after 3 years. 

What vesting schedule do they want in the surviving plan?

Why not use that one and follow the change of vesting schedule rules for anyone who the surviving vesting schedule isn't better in all years?