I've installed the 1.1 patch without any adverse effect to my save games. One way to be sure tho is to do a backup of all saves, install the patch and see if it works okay. As far as i know though the patch is mainly for multi-player sync problems with only a few balancing issues for single player. These balancing fixes should have no impact on save games, you can tweak units all you like during a campaign.
*Ringo*
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