This is an API breaking change. If you previously experimented with media assets, you will have to rebuild your Blueprints, UMG widgets, etc. If you don't want to lose your existing Blueprints, you can add the following line to your project's Engine.ini, which will map the old class name to the new one (you will still need to replace existing MediaAsset nodes with MediaPlayer nodes):
[/Script/Engine.Engine]
+ActiveClassRedirects=(OldClassName="MediaAsset",NewClassName="MediaPlayer")
[CL 2291399 by Max Preussner in Main branch]