joining a analyst program from middle office

Maybe someone with personal experience in a current bank can offer some specific insight. (I know similar topics have been posted, but still haven't found specific info)

I was curious to know if you are in a middle office role, finance/compliance/corp-strategy or whatever, how hard is it to join say the new analyst class for a front office role???

Can someone explain the process of doing an internal transfer? I know some banks require you work a year or two before being considered for internal transfer. But what are the barriers between joining a new class, is HR very against this? I'm just surprised when I hear people talking about how hard it is, I mean there is a new class of 100+ kids coming in every summer, how hard could it be to get a senior mentor to give a recommendation to HR for you to join the new class? (People have mentioned that when a senior guy gives an "ok", that HR has no say in the matter. )

...From another perspective, If you have a mentor or friend of seniority, what sweat is it of his back to let you/push you into a new analyst class, chances are that you won't end up in his group/desk/or whatever, so he won't be accountable for your success/failure anyways. The new analyst class is like a sea of new random people from various schools, cities across the nation.

Maybe people who are familiar with the process or perhaps have successful made the transfer from middle office to front office can share specific details, obstacles, and corporate procedures.