Yes, you have to, whether it’s Surefire or OSS.
You have to get two 19 mm wrenches to do the job (loosen the locking nut first to get the FH off). Use the locking nut to lock the new FH up against it. No, it’s not optimal, but it’s what IWI recommends.
IWI US Expert's Corner: Tavor Flash Suppressor Removal
https://www.ar15.com/forums/ak-47/Removing-Galil-ACE-Flash-Hider/92-166788/
The video is for the Tavor, but I emailed/called them to confirm this for the Galil ACE. It’s the same process.
I have a 7.62x39 SBR Galil ACE with a Surefire adapter. I had a gunsmith do the timing for me, and it works great. For my 16” rifle, I installed the OSS FH on myself without an issue, since it doesn’t need to be timed. I used Rocksett of course. No issues thus far, and my suppressor alignment rod looks great for both flash hiders.