As the others have mentioned, there is no need to reinstall.
There is also no need to have two huge separate game directories.
If you are concerned about being able to join different servers until they all move to 4.09m... a simple bat switcher is easily made to switch between 4.08m, 4.09b1m and 4.09m.
You can use the simple one on this site if you like and/or modify it to your needs.
http://battlezone.webhop.net
I'm sure other fancier switchers will be out soon as well.
S`
HB