I ran into this problem, too. Couldn't figure out why some of my store listings kept ending after 30 days and didn't. I haven't figured out how to get that number of days to change automatically, because selecting an Item Profile overwrites all the info in the listing you are trying to change.
What I do is copy the detail info., click on the listing format type to change it to Store, change the Item Detail format to one of my "store" types [which has the Good Til Cancelled selected], paste the item info, and go from there.
Maybe someone else knows of a better way? Love to hear it!
I think Auctiva is great and I am very pleased with it, especially the templates and the extra pictures. Even if there isn't an easy way to get the GTC selected on a changed listing type, I'm happy.