No, the 20 points that were borrowed have to stay in the 2008 use year. Borrowing is a final transaction. The borrowed points expire on 9/30/2009 if they are not used before then.
The 100 points from your 2008 use year that were used for the July 2009 reservation can be banked if you cancel the reservation at least 31 days prior to arrival and call MS to do it on or before May31, 2009.
Try thinking of it this way:
Current use year points are
green. (The use year of your vacation dates is the current use year for that vacation).
Green points turn
red after the banking deadline or if they are banked or borrowed.
Once a point is
red, it expires at the end of the use year unless it is used. You can't turn a
red point back into a
green point!
Red points are "use 'em or lose 'em".
So your July 2009 reservation was made with 100
green points and 20
red points. If you cancel it in time, you can bank the 100
green points, but must use the 20
red points before they expire - which is at the end of the use year, 9/30/2009.