A simple migration plan
Do not start by rebuilding every tab in a new tool. Start by working out what the spreadsheet is actually doing, the shift patterns, availability, leave, coverage checks, color codes, formulas, and the manual workarounds living in one person's head. That hidden logic is the real thing you are migrating.
Audit the spreadsheet and hidden rules
Clean employee, role, and availability data
Move one team or location first
Run one parallel schedule for confidence
Retire duplicate files after the live schedule works