The way to get to older versions of Skyline is by clicking the "Archive" link on the page from one of the "Unplugged" installer pages.
Here is the page for 32 bit unplugged installers:
After you click the "I agree" button you will be able to click the "Archive" link that appears.
Current versions of Skyline can always open Skyline documents that were created by older versions of Skyline.
If you are using the current version of Skyline and you need to give your document to someone who is using an older version of Skyline, you can use the menu item:
File > Share
to create a .zip file containing a document that can be opened by the older version of Skyline.
I am not sure I understand why you think you need to go back to an older version of Skyline in order to use the same transitions that someone else is using. Normally, the reason that people need older versions of Skyline is to exactly reproduce the numbers that the older version of Skyline was calculating. (Also, if you're in the middle of spending several months analyzing a large complicated dataset, that would be another reason not to want to upgrade).
Also, we always recommend that people use the current version of Skyline. Do you know why your collaborator does not want to use the latest version (20.2) of Skyline? Please let us know if there is something wrong with the current version of Skyline which prevents them from wanting to use the current version.