You can leave 13.1 installed and install 17.0 in addition to it. Each version will live independently in its own folder.
If I remember correctly - because these versions are so far apart, Toad 17.0 will not import your settings from 13.1 automatically, but you can do that manually under Main menu -> Manage User Settings. After that, each version will retain their own copy of settings moving forward.
We will be releasing a patch to 17.0 in December or January. A few significant bugs have been reported and the patch will address them. You may want to wait for that to be released before you update. Other than the patch items, 17.0 is so much better than 13.x. There is a long list of bug fixes and enhancements between these versions. I always feel like I am entering the stone age when I fire up an old Toad.
If you want to look at the change log for version 17.0 you can start here, then at the end, there is a link to 16.3's list, and so on.
-John