I'll be glad to.
Some time ago I went to the Apple Store to purchase a MacBook. I wanted a higher-spec model than what they had in stock, so I asked about getting more RAM, a bigger HD, and faster processor.
The sales associate told me that the only upgrades they perform in the store were RAM upgrades. If I wanted a bigger HD or faster processor, I'd have to order online.
But to (hopefully) answer your question, I was specifically told that if I wanted to upgrade only the RAM, they could do it right there in the store.