I think the only chance that you would have would be to search out a line in the logs that reports the memory size, which would be part of the initial boot. Then, you would review previous logs until you find one that's different.
And, it won't tell you WHEN it was changed, only that the memory report is different at some point.
One fallacy to THAT would be if your logs have been cleaned out during a routine system maintenance (I usually clean out old logs every month or two), and your hardware change was older than your oldest logs.
Why would you need to find out about a RAM change?