jwhands makes a valid point. Lincecum wasn't supposed to come up as soon as he did last year, but he was so dominant in the minors, he had nothing left to prove there and the franchise was wasting its time and his time by keeping him there.
The other thing to keep in mind is that Braun was called up when he was 24 last year, and LaPorta is 23 so not much age difference obviously. I'm not saying their criteria is based on simple things like that, but I am trying to say I don't think it's a huge longshot. We're also seeing that the Brewers' line-up is pretty flawed so far this year, so they might welcome the offensive boost if that continues.
I'm no expert, but just throwing my two cents in... someone else out there might have a better idea of the Brewers' long-range plans for LaPorta (maybe Ender does or anyone who's been reading up on it or listening to interviews with the Melvin or Yost?).