ASG-220 early rev - can't upgrade to 8.x?

Hi there. I have an older ASG-220 with 512MB of RAM running 7.512 that has a current license and subscription. It's old enough that the serial number has a dash in it, which I have not seen anywhere else. I tried upgrading it to 8.x via Up2Date, and it came back and said it didn't have enough memory. Any ideas on what I can do to install 8.x on it?
TIA
Brian