Reagan was a fairly effective convert, as was Phil Gramm, Dick Shelby, and a long list of others. Think David Horowitz. There are a lot of folks brought up in the Democrat party who come to see the light. Converts often make the strongest partisans.
At this point, I don’t think casting aspersions on Bell’s motivations and character is helpful.
If Bell is known to have consorted with extreme leftists, it is a fair question to ask of him whether he has “refudiated” them.