I am working on the Raspbian Buster OS.
To move from Buster to Bullseye, I have to make the following changes.
Updating source list.
sudo nano /etc/apt/sources.list
Replace any occurences of buster with bullseye.
Update packages.
sudo apt-get update && sudo apt-get upgrade -y
I would like to know when will the Debian Bullseye compatible Raspberry Pi OS be expected to be released?
No one can answer your question - it is unanswerable, just as the question, "When will the universe collapse on itself?" You've asked for an opinion, not an answer. That said, it seems likely that the bullseye release will happen before the universe collapses.
With respect to the "in-place upgrade" you've referred to, it may be a good learning experience, but unlikely to turn out as well as you'd wish. Raspberry Pi OS has never provided a sanctioned/guaranteed "in-place upgrade", and it seems highly unlikely they will start now.
If I were determined to try a bullseye upgrade in advance of the official release, as an "in-place upgrade", I would read RonR's post, and try his upgrade script. Why? He has something quite rare: credibility.
I would suggest very soon:
"Software seems to be the name of the game in 2021. For starters, Raspberry Pi is aiming to migrate the platform to Debian 11 by the middle of the year." I found it on this webside. Personally I also tried to upgrade it with the sources.list, but it did not work well, e.g. the sound is strange.
