Yep, just pull the release valve on the receiving keg every so often.
My advice is to buy one of these: http://www.northernbrewer.com/shop/smartstrip.html
It made it a lot easier to figure out how much beer is in the smaller keg. The first few times I did it without the SmartStrip, I opened the lid a few times to see the level since I thought that was a lot better than overfilling the small keg. You'll need to cut it to fit the 2.5G keg. Another option is a scale, but this is a lot easier.