The locking/unlocking mechanism presently is quite frustrating.
Luckily there is a really simple option to unlock all and that is possible due to the open fileformat.
if you were to open the PCB file in any text editor (notepad, vim,…) and do a global find/replace for locked it will remove the locking.
NOTE: if you have named something (layer, trace…) as locked it could cause issues, in that case the wider
locked (layer replaced with (layer should work.
Always keep a backup