Currently basic users can go to another users task and merge their personal task into the other users task. This does not then give them permissions to edit the other users task they merged into. At least, this doesn't give basic users a way to gain edit permissions for tasks, but it does let them merge all of the details from their task into another users task.
An example of this is https://code.alephobjects.com/T194
I created https://code.alephobjects.com/T193 on my basic user account. I then went to T194 which was created by Nick on his basic user account. From T194, which my account did not have access to edit, I was able to use Merge Duplicates In to find my tasks, T193, and merge it into T194.