There is a lot of out of date information about this process.
I recently accomplished this on a wp8.0 lumia 521.
first, I did not need to first install lumia pusher.
second, you need to clean off your old maps before doing anything with storage check beta. If you leave old maps on your phone, and do this process, you will never be able to remove the old maps. (unless changing the parm back to the phone storage, deleting them from there, then changing the parm back to the sd card works)
third, install lumia storage check beta. I could still find this in early september 2014.
fourth, using lumia storage check, change the parameter to put the maps on the sd card.
fifth, download the maps you want.
Since I did this a few days ago, I don't know how to exactly state all the buzzwords.
One fact is you don't need wp8.1 to accomplish this. My phone was updated to 8.0.10517.150 before I tried this process. Some webpostings say you need to be on 8.1, this is not true.