It is MUCH easier and safer to just piggy-back on top of the existing chips, for the first (added) layer.

If you have a good heat gun/pencil, it would have been much easier (at least for me) to reflow 16MB chips instead of piggybacking 8MB chips. The parts should come off easily, and since the pads are so big, it wouldn't be hard to add solder for reflowing.