I'm sure someone with more experiance with PPCS5.5 will be able to give you some other advice, but I'd like to share mine. There is software called Magic Bullet Instant HD V1.2. It does a very good job of taking SD files and giving you a better final image than the SD file. You are able to set many different sizes and resolutions. I had about 50 hours of SD video and it was worth it to me to be able to view them in higher quality. You can download a trial version of the software to see if you like it.
Uprezzing an SD clip would be the inferior solution to replacing the SD with the HD originals.
Try it. Make a copy of your project, recapture from HDV, move the folder containing the SD sources (or rename it) and you be asked where the footage is. Of course, the newly captured stuff should be named like the stuff you had before.
I am really interested if that would work...;)