I did several memory board upgrades without a hitch (knock on wood).
A couple of suggestions:
1) Based on your pictures I see that pads adjacent to the CPU ones are not completely flat. That could create problems while soldering CPU pads. I absolutely flattened entire area where the memory board would sit on. It takes a while but is a good way to ensure excellent connections from the get go.
2) In order to make sure you placed your board right the first time is to put small amounts of solder in several spots across the area where the board would be placed. This way you create small semi-circular spheres that will lock your board in place instantly. A better way is to make these solder spheres for all of the memory board holes so you never have to excessively heat the underlying pad.