Putting a plane back into Phase I often requires a letter to the local FSDO and a new test area along with a written response from the FAA (or DAR). Your operating limitations will specify what's required. Some just require a log book entry but many require more. Even if you just have a log entry, you have to have a phase 1 test area agreed with the DAR or FAA. That's super easy if you still live where the original phase 1 was done and have the paperwork.
I had to do this for an inverted oil system on my old Skybolt. The airplane had a Christen system originally that was removed. I asked the local FSDO, they said going to a Raven inverted oil system required a new 5 hour test period for the major alternation.
It took me about 6 weeks to sort the paperwork with the FAA between negotiating a new test area, getting a new set of operating limitations, and then they came out to the hangar to do the final paperwork. The folks were nice and training a new person so maybe they were more thorough than normal. A DAR likely will be quicker.
An easy link to 14 CFR Part 43 appendix A defines a major alteration
I did the paperwork before installing the oil system to avoid down time.
Check to make sure your insurance covers Phase 1 operations.