From manjaro wiki:
How do I go back after changing to one of the testing branches?
Going back to the stable branch is easy. All you have to do is to repeat the above, and use stable as the branch value.
Be aware that after switching to a more stable branch you will receive messages from pacman, informing about newer packages installed than available in the repo. Don't be alarmed as the situation will resolve itself when the packages reaches your current branch.
On testing kernel ix 5.xx
On stable it's 4.xx LTS
What is going on when moving from testing to stable?
System will wait till stabke 5 LTS or it is for me to decide what do do?
After changing branch to stable can I install older LTS kernel or should I wait for 5 LTS/