Technical Specification/Solution

Hi,
I have been tasked within a big project to write a technical specification/solution for a new system that will be replacing the old one. In fact relevant department requested the IT department to provide a guidance/solution in upgrading the current system. I am not sure what kind of report it would be. feasibility report/functional spec ??
If there’s anyone who has done this before or a template I can use as a guideline will be most appreciated.