I wonder how the offer of conversion would be handled dealing those that in prison. I doubt the public would denounce such a thing as rewarding criminal behavior. I just don't see it much in Conversion Bureau stories. What do you guys think?

5417897 They'd convert them as cheaply as possible when it was convenient for the government to do so, without asking their permission, because as prisoners they've given up their civil rights. (well, not all rights, but they did lose the right to refuse medical treatment in most cases)

Then they'd keep the ponies in prison for the rest of their term.

5417897 I'd imagine that in settings where Equestria is expanding to cover Earth (or Earth is otherwise soon to be uninhabitable), prison inmates would be converted in prison shortly before the local area became untenable for humanity, and then released; in settings where Earth and Equestria will reach an equilibrium, conversion would be tied in with existing parole systems, and would either be available only under certain conditions, have strings attached, or both.

In both cases, any limits would likely come from Humanity alone.

It's cheaper than keeping them in prison. I can see mandatory conversion for lifers as a way to balance budgets.