I have been able to upgrade one card in my 1975 Topps PSA Graded Cubs set. Recently, I purchased a PSA 9 copy of card #39 - Andy Thornton. This card replaces the PSA 8 version that I had. The PSA 8 version of this card was posted on December 2, 2009.
This also is a good time to undate the current status of the 1975 Topps Cubs - PSA Registry set. My set is currently #7 on the current finest list at the PSA Registry. My set is 77.42% complete with a weighted grade of 8.22.