In theory you should be able to leave the plugins folder you have now in tact, and just create a shortcut to it in the new plugin folder.

In reality, some of the stuff may still have to be re-installed. The only way to know for sure is when the time comes.

The other option would be to save the old folder in a temp area, then after PS 7 is installed open it, select everything in there and copy it to the new one. Effectively this does the same as above, but eliminates an extra folder as you can delete the old one after you are done.