Hi there - thanks for reaching out for clarity. 
Any content previously downloaded through Quixel.com or Bridge would still be governed by the license that was in place when it was acquired. So you’re allowed to use previously downloaded Megascans assets in your past, current and future projects under the license terms that were in place when they were acquired.
Please let me know if you have any other questions. We’ll be publishing buyer guides when Fab launches and they will contain more info around licensing, past purchases, etc.