You don't need to uninstall Poser 6 at all. You can have them installed alongside one another, since they are in different Applications folders. They will also use separate preferences files, in your /Library/Preferences folder.

If you have lots of content in the Poser 6 main Runtime, you can create a new Library in Poser 7 and point it to the Applications/Poser 6 folder, and all that content will be available to Poser 7.

The only problem with this might be if you install Poser 7 SR3 (which I would strongly advise you do) then you might get some problems due to duplicated content files across the two Runtimes. In this case, it may be better to create a completely new external runtime for Poser 7 and transfer old content across to it as required, without creating duplicates.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)