I tried adb sideload using the /e/ recovery (without wiping) to flash 2.6.4-t and it reported ‘Signature verification failed’, upon flashing anyway it seems to succeed but is now stuck rebooting into the system (hopping e icon).
wiping is not as formating, it will wipe /data content excepting your internal storage,
when formating delete all the /data content including your internal storage.
Thanks for your help. TWRP 3.7 was able to go back to 2.5s, but it couldn’t start due to encryption. Tried the ‘file system’ trick from the above article, which circumvented the encryption, wiped the data partition then adb sideload 2.5s
The phone is now working again on 2.5s but it is now like a fresh install (data/apps lost).
Maybe I should have removed the encryption before wiping data… not to worry!
recovery-e was a better candidat to revert to 2.5-s because of the encription
better solution would be to use adb push pull throught the recovery
to save the internal storage via command line (not 100% sure)
before wiping or formating /data