I'm trying to upgrade my Pix 515e, ver 6.3.1, to PDM 3.04. According to Cisco I need the DES activation key and the 3.0.4 software. I have the activation key, however, when I try to upload the PDM software, I get an error message "Not a PDM file" then deletes the partial loaded file. Cisco recommended to use a different TFTP software. I did and I'm still getting the same error message.
I found out that the pix had a filter for active x and java applets. Everytime I downloaded the PDM software the pix strip down the java component. I removed the filters and downloaded a fresh copy of the PDM software. This solved the problem. But I think you are right is always safe to upgrade to code 6.3.5.