What you migrate is all the PPX reports. I am currently doing migration. I got all the PPX reports from the PPSROOT into a folder. Organized all the reports into a structure that is same as structure on Upfront. (Or you can create a package *.dmp). Then follow instructions from migration tools document and run migratetoc8 and deploytoc8 commands. Then import the deployment archive.
After that there is a cleanup task. I opened each report on Cognos 8, checked the package, report etc. Had to do many calculations again.