I thought about this question after dreaming I was a Sabbat vampire and for some reason I rebelled against them and allied myself with Camarilla (yes, I have crazy dreams). Thinking about this, I found this question, which talks about the possibility of a vampire being a sleeper agent inside the other faction, but that is different than my case - I'm talking about one actually leaving Sabbat to join Camarilla.
For what I remembered, Sabbat vampires have a kind of blood bond after being embraced - according to White Wolf wikia, there is a kind of weak communal blood bond called Vinculum, which "generally makes pack members supportive of and loyal to each other, though it does not produce the extreme reactions of a full blood bond". The Vinculum can happens during the Vaulderie, a "blood rite practiced by the Sabbat to break any existing blood bonds and unite the pack members closely together"; Vaulderie happens immediately after the Creation Rites to turn someone into a True Sabbat.
So, for what I understood, this means that any True Sabbat will have a kind of blood bond with other Sabbat members (i.e., it's pack). Since Vaulderie break former blood bonds, theoretically a Camarilla member can join Sabbat.
But what about the opposite? How can I justify a Sabbat Vampire leaving its sect to join Camarilla, if it's even possible, both in mechanical and setting aspects?