> elogd 3.1.0 moves all elog entries into year-named subdirectories. this feature makes it incompatible with older elogs and so should be clearly mentioned in the documentation,
> in the release announcement and in the release and migration notes. K.O.

That feature is one of the main reasons why the version jumped from 2.x to 3.x. 
A free tip: changes in major revisions do indicate some kind of incompatibility.
But yes, the release documentation by bitbucket is not really that useful: 
it is difficult for me too, to find out what changed with new releases. 
I have to admit here, that I haven't read any GIT tutorial yet.
By the way: you are welcome to contribute to the release documentation!

On your actual problem: to go back to a former version of ELOG you can simply
- stop elogd 3.X, 
- move all entries from the sub-directories one level up, and 
- start the 2.X version of elogd.

I wouldn't really call this an "incompatibility", would you? 
At least you can easily go back without much trouble.

Cheers
Andreas