Hi wanderer33,
There are several threads with info on this if you do a search.
Having been through this, I found I had to get a new mortgage and transfer the title, which means doing pretty much the same sort of stuff as if you were buying again. i.e brokers, solicitors, valuers etc all have to become involved. As NorfBank said you will have to be able to afford the mortgage on your own merit.
I didn't have to pay stamp duty on the purchase of her share, but I am not entirely sure if that would apply to every type of situation like this.
/MN