Some people have said their saves work okay-ish, though with some odd province assignments such as colonial nations getting provinces in Lithuania, but for me the game crashes when I try to load an old save.
In order to make a backup copy of an old version:
1) Revert to the old version you want in steam by right clicking on EUIV in your library, selecting properties, selecting the beta tab, and selecting the version you want from the drop down menu.
2) Copy and paste the entire EUIV folder in ...steam\steamapps\common to a new folder of your choice.
3) Create a new text file in the copied EUIV folder and name it userdir.
4) In the userdir text file type the file path for where you want your save games, screenshots, and mods to be saved. For me it's C:\Users\Admin\Documents\Paradox Interactive\EUIV 1.5.
5) You can either manually create the folder or simply start the game once and save a game then quit.
6) Copy your old saves into the save games subfolder of your userdir folder.
7) Create a shortcut to the EUIV launcher in your back up EUIV folder. Launch the game with that.
8) Go back to Steam and unselect any betas (or select the 1.8.1 beta) from the EUIV properties menu. Launch 1.8.x through steam.
