I finished Sustained Power Build in early Feb and was trying to decide between re-basing with SSBI or SSBII. I went SSBI and was glad I did. It took me 3-4 week to feel like I fully shed the fatigue from SPB. My FTP assessment at the end of re-basing with SSBI was the same as the beginning, so no gainz lost.
Also, if your FTP increased, re-basing with SSBI might be a good way to recalibrate your internal RPE meter to your new threshold. One thing I find difficult with a big FTP bump is realigning my assessment of RPE with the new power targets. Old threshold RPE might now be at sweet spot power, 105% FTP RPE might now be new threshold power, and it takes a little while for my brain to adjust to the old 105% FTP RPE being held for much longer.