How to change Map or Blueprint to older version?

I used the newest source code master branch, its 4.8. Now i want to change to the release branch (or to the non source code version), this is 4.7.2.
BUT, if i go from 4.8 to 4.7 i cant use all my Maps and Blueprints (which i saved with the 4.8 version).
Someone has an idea to solve this? How can i use the release version?

For example i get these messages, while the project starts:

Failed to load map!
../../../../../../GameDev/UnrealProjects/ProjectTest472/Content/Maps/MainLevel.umap is a UE4 map [File:v443], from an engine release newer than this [Cur:v434].

There are no mechanisms to reverse the package migration process to a newer version. While in some cases it might be loadable in the older version, generally, the forward migration of the asset will have structurally changed the file in such a way that it is no longer compatible with older engine versions.

Thanks Marc, too bad, now i have to stick with the master branch until 4.8 releases.