There is no need to remove the present install of Felicia. You can install Gloria right over Felicia. When the installer asks how to partition the drive for Gloria, Choose "Manual (or I think it may be Custom in Gloria) Install". When the partition table comes up select the existing ext3 partitions containing Mint 6 and check "Edit Partition". Leave the partition size the same, Check "Format", Select the partition label (/, or /home) to correspond to the Mint 6 partition being replaced, select ext3 or ext4, and click "Okay". The Mint 7 installation will overwrite the Mint 6 installation with no problems.
BTW, you don't have to touch your swap partition. Swap is the same for all Linux installations.
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