This seems like a common problem with fire alarm systems. I work in a University job as an electrician and we do our own FA upgrades. The first step is always finding out what is existing, not always easy, and laying out the new system. All upgrades are analog addressable,and we use as much existing wiring as possible. The SLC bus splits out to feed the new detection devices, and the key here is a drawing of adresses and phycsical locations. This really helps with permitting,programming,cut over, and troubleshooting. Our first physical step is one or more terminal cans set up to interconnect with the existing wiring. If space permits, we mount the new FACP next to or near the existing,power it up and program for the change over. The actual conversion in our case means changing of monitor devices since we are changing manufacturers. This is what takes time. Example, our performing arts center has about 400 detection and monitor devices, which need to be changed. Since we can't afford to shut down the FA, both systems will be running with each doing some of the detection as the change happens.
I can't stress preplanning enough. It is tedious but well worth the effort. Labels, layout, and room to wire help a lot.

Good luck