How to downgrade to a previous game's version?

Yes

Right-Click EU4 in your Steam Library. Select Properties, select the Betas tab.

Click on the drop down menu at the top, which should say "NONE - Opt out of all beta programs". Here you can select all the old versions to play, all the way down to 1.4.

Accepted answer is no longer correct as of 1. october 2018.

To change version you now have to do the following steps:

First, get the code to access beta-versions in Steam

1. Go to https://accounts.paradoxplaza.com/games and log in (or create) your Paradox Account.
2. Scroll Down
3. Find "Access Earlier Versions"
4. Click "Show"
5. Copy the code

Then revert to older versions in Steam.

1. Go to your Steam Library and select Europa Universalis
2. Right-click and select Properties.
3. Under the “BETAS” tab.
4. Enter the relevant code retrieved from Paradox Plaza and click “CHECK CODE”.
5. Select the version you wish to play and wait for Steam launcher to finalize the switch.

If the previous step fails, exit the game, right click properties, local files, Verify integrity of game files and wait for all files to be verified.

More info on Paradox' forum