Is there a bacterium that became a virus?
This virology site has a post about a 2017 paper about membrane-vesicled plasmids that act in ways that are theorized to be precursors to how viruses work:

It is likely that the plasmid-containing membrane vesicles are precursors of what we know today as virus particles. It is thought that viruses originated from selfish genetic elements such as plasmids and transposons when these nucleic acids acquired structural proteins (pictured; image credit). Phylogenetic analyses of the structural proteins of many enveloped and naked viruses reveal that they likely originated from cell proteins on multiple occasions (link to paper).
The membrane-encased Archaeal plasmid seems well on its way to becoming a virus, pending acquisition of viral structural proteins.

Though plasmids are not viruses and are not bacteria, bacteria are hosts for plasmids, and plasmids add indirectly to the genetic constitution of bacteria. Expression of plasmid genes can help bacteria survive hostile or extreme environments, and they reproduce along with bacterial chromosomes. Via the Wikipedia link:

Unlike viruses, which encase their genetic material in a protective protein coat called a capsid, plasmids are &quot;naked&quot; DNA and do not encode genes necessary to encase the genetic material for transfer to a new host; however, some classes of plasmids encode the conjugative &quot;sex&quot; pilus necessary for their own transfer.