Fairphone 
switch from /e/OS official T (A13) to /e/OS community A15 in one go. Details below.
Not the recommended way
Starting point:
- FP3 on e-3.0.4-t-20250709507786-official-FP3
- Bootloader status: locked
- Use developer options: activated
- OEM unlocking: activated
- USB debugging: activated
- no change attempted to bootloader status
Attempts of local updating
- Local update to e-3.1.1-a15-20250830521751-community-FP3: failed
- Local update to e-3.1.1-a14-20250901522231-community-FP3: failed
Attempt of ADB Sideload via /e/ recovery 3.0.4
- ADB Sideload of e-3.1.1-a15-20250830521751-community-FP3
- Log on display:
Now send the package you want to apply
to the device with “adb sideload ”…
Supported API: 3
Finding update package…
ERROR: recovery: failed to verify while-file signature
Update package verification took 114.5 s (result 1).
ERROR: recovery: Signature verification failed
ERROR: recovery: error: 21
Installing update…
Step 1/2
Step 2/2Install completed with status 0.
In the course of the process right above, I had to respond “Yes” to the message:
Signature verification failed – Install anyway?
Booting into system from /e/ recovery 3.0.4 succeeds.
Finishing line:
- FP3 on e-3.1.1-a15-20250830521751-community-FP3
- Bootloader status: locked
- Use developer options: activated
- OEM unlocking: activated
- USB debugging: activated
Final observations and comments
- I did encounter a bootloop after the first boot into the new system and first reboot, but upon later boots, this has not (yet) occured again. I might have entered recovery and bootloader (no change attempted) again in the meantime, but do not remember the exact succession anymore.
- The (regular) reboot takes a relatively long time (40 seconds) on this FP3 now (appears dead for a while until it “wakes up” again), on my daily driver FP3+ it takes roughly 22 seconds.
- If you want to do the same operation at your own risk, make sure to download e-3.1.1-a15-20250830521751-community-FP3.zip, not IMG-e-3.1.1-a15-20250830521751-community-FP3.zip