a2 means alpha 2. It is an alpha version, so not final and will probably change a lot. Upgrading to bullseye is possible changing the repositories, but it will NOT yield the same result as the current alpha 2, which has different configurations and applications.
See this post on upgrading fro 19.3 to testing: https://www.antixforum.com/forums/topic/future-antix/#post-46898
Simply change “testing” for “bullseye” once you are finished and you would achieve similar results.
Note: This process may break your system, so back up everything or first test it out on a Virtual Machine.