Please elaborate on how the optimal plan is to get someone to snitch *after* they have gone through jury trial, been convicted, and sentenced. Are you even aware of any legal mechanism to vacate a conviction and sentence, not based on any legal error like in an appeal, but based simply on the agreement of the parties due to subsequent cooperation by the defendant? There are laws and procedural rules that govern this kind of thing, you know.
Even assuming for purposes of argument that such a thing was possible, it makes zero sense. Cooperating witnesses are always subject to credibility attacks by defense attorneys due to the benefits they received for cooperation. It's hard enough for a prosecutor to bolster a witness's credibility when they received a plea to a lesser charge due to their cooperation. This challenge would be exponentially greater if a witness was literally convicted and sentenced to life in prison and then received a do-over and a few years because they decided "oh wait, yeah maybe I do have some valuable information to share..."
I'm not trying to be flippant here, I just really don't understand this line of thought, to say nothing of my inclination that it is legally, procedurally impossible in the first place.
Agreed - I've seen it happen post jury trial conviction before, but PRE-sentencing. Though I guess it is procedurally possible post-sentencing since someone posted a link. But like you said in paragraph 2, the credibility of incentivized post-conviction testimony lessens tremendously. When the defendant has been tried, convicted, and sentenced (presumably) to life - what incentive do they NOT have to try anything to better their situation, including concocting a story inculpating others? That's what the defense attorney in me would say.