What does a real "modernization" look like for a 250-EE company still on legacy payroll?

We have ~250 employees split across 4 states, currently running payroll through a legacy provider that has been fine but slow on benefits admin and useless on reporting.

Leadership wants us to modernize. I want a tighter definition of what that actually means at our size.

For those who have done it: what did you change first? What surprised you about the implementation? Was the migration worth it within the first year, or did the value show up later?

Happy to share more context if helpful.