I didn't try this, because I clean installed 8 CP. But, there was an option in 8 CP to keep Windows settings, personal files, and apps. The other options were 'just files' and 'nothing'. I suppose if the CP to RP allows to keep files that would be ok, but it would be great if I could keep the apps too.DP > CP upgrade was not officially supported.
I would not recommend it personally, if you really want to test it properly.
Since it risks to carry over bugs from CP > RP
These kinds of bugs occurring during such an upgrade would not be relevant for the RC testing stage...