Seems like a conversion of convenience.
Hard to say if it's a conversion. Not everyone has a position on everything. As governor, he had no direct influence on immigration policy, a federal function. More likely, at the time, he offered up the usual pablum that sounded like the consensus within the GOP, voters, pols and donors alike.
Now that he's doing a White House run, he's clearly up for a spot that will play a major part in immigration policy. And that's led to a much more careful examination of what that policy should be. In talking to people, my guess is that he's discovering that that there is a great divide separating voters, who are aghast at the deluge, and both GOP pols and donors, who seem to think we need to open up the floodgates.
There's nothing convenient about Walker's current position. By suggesting that legal immigration levels be rethought, he is opening himself up to accusations of nativism and racism, and that's just from fellow GOP pols. Then there's the adverse reaction from wealthy donors whose principal concern is keeping labor costs low by importing unending streams of foreign immigrants. These donors may start treating him as the party leper.