It can be done, and I've done it myself. Keep in mind this only works with a clone of the same route, meaning a save as and will not work with a totally different route.
This has to be done in Content Manager, and I recommend cloning the session you want to use, rather than using the original.
1) In Content Manager, open the route for edit in Explorer. This is an option in the Edit menu, or right-click on the route and choose the option from the menu.
2) Using Notepad, edit the config.txt file, and change the KUID from the old session to the one you want to use.
3) Save the config.txt
4) Close the folder
5) Commit the route back to the database by highlighting the route and pressing CTRL+M or by choosing the option from the edit menu. There maybe a warning about a missing dependencies. Don't panic, yet as this will go away once the session is updated.
6) Repeat the same in the session in Content Manager. Change the route KUID in the config.txt file to correspond with the KUID for the new route.
Make sure both the route and session are committed, and check for errors and warnings. If there are any errors, check that you didn't make a typo.
I hope I got this right. It's a bit past my bedtime and I'm a bit groggy.
John