Convert Rotation Mode

Change the rotation mode of the selected bones and preserves the animation or poses you already made.

Select one or more bones in Pose mode, select the rotation mode you want them to use, hit the Convert! button.

It will automatically scan through all the keyframes of the selected bones in the timeline.
You can customize the sidebar tab in which to spawn the addon panel.

If you are unsure what rotation mode to use, look at the rotation modes cheat sheet!

Please not these are merely suggestions, what you will actually need may vary from one rig to another or even from one animation to another.

Note that there are two main coordinates system for bones. Blender uses Y down, so this is most likely the one you will use. But you might need to look at the X down in some cases like exporting to softwares that can only ready X down.