Quote - Ok, then when I have copied the Poser 7 runtime to Poser 8 ...

This is important, don't copy the Runtime into P8, bad things will happen.

RC suggested making a copy of the directory elsewhere, then connecting that directory in P8 as an external Runtime.
You can then remove the remainder of your P7 installation.

However there really is no need to uninstall P7 (it won't save that much space), you can connect the Runtime directly in P8.