Yes - recordings scheduled on Auto HD enabled channels will record in HD when Auto HD is turned on. However, because HD programs take up much more space on your DVR than standard definition shows, you may notice that your DVR storage is getting fuller faster than it did before.
If you’re concerned about maintaining free space on your DVR, just turn off Auto HD. Go to the Settings menu by pressing the Settings button on your remote. Scroll using the right and left arrow keys until High Definition is highlighted, then scroll using the up and down keys until Auto HD is highlighted.