Nope. You don't need to do anything at all with your old runtime folders.

First install something for you to add.. if it's a daz item, install it anywhere, with a unique folder name. (say install Freak into a folder called "freak").

Now you open Poser, and use the up folder icon in the library palette to get the the top level, where you see the "poser6" folder.
At the bottom right, you'll see a checkmark, and a "plus" sign. Click th eplus sign to add a new runtime, and ten browse to the new "freak" folder you created. (NOT the runtime IN that folder!)
Now you'll see a Poser 6 runtime folder, and also a reak runtime folder. select the freak one, and you have access to the content in that new runtime. To access the stuff in your original one, just use the "up folder" icon to get back to the top level, where the P6 and freak runtime folders are, and select the Poser 6 one!

After you do it ONCE, you'll be dumbfounded why the hck you never did it before!

Also, if you want to get rid of content you don;t want to use for a while, FIRST removed that new runtime folder with th eminus sign once it's selected, and THEN just delete the install folder you did manually. (Removing the folder first outside poser will confuse it when it looks for it.)

My explanation might not make much sense, bu tthere are MANY explanations here about adding additional runtimes, and I'm sure they might be clearer then this explanation.

Just use the search option.

Way too many people take way too many things way too seriously.