No problem. I did it that way. Just sideload the zip in recovery without wiping the data.
No problems at all. Everything’s running fine.
Just two things if you update to 3.0 that way:
First
You have to clear cache and data in bluetooth app after first start in 3.0. Otherwise if will crash permanently.
Settings/apps/ - show system apps - bluetooth apps
Second
Enable developer settings
Enable “deactivate bluetooth a2dp hardware offload”
Reboot
Otherwise bluetooth audio is not clear
At the weekend I updated my FP4 from iodeOS V2.5 to V3 Over the Air (OTA) with no issues. All my data was intact, only the bluetooth cache needed to be cleared to enable bluetooth. The only extra work was ensuring I backed up my SD Card as Android 12 insists on reformatting my SD Card before I can use it. (I have reposted this on the Fairphone forum too)
Updated Over the Air (OTA) from V3.0 to V3.1. No data loss, no issues seen so far, smooth update. Used WiFi to download. Biggest imeadiate improvement, which may be a placebo, was the perceived improvement of the quality of the pictures using the Open Camera app.