SAVE THE ANIMATION
CHANGES CAN HAPPEN
Very rarely and close to never we change the name or position of a controller once it has been released. If there is a mistake in a rig and it requires change we will in almost any case avoid pivot positions and naming changes to prevent our clients to lose animation. Problems will be solved with adding mechanics instead of changing current and already active articulation deformers. However if something is fundamentally wrong we must in some cases move a pivot which will mean you will have to change your animation slightly, keep in mind this is done so your character will in the end result look better.
REPLACING REFERENCE OR NAMESPACE
If you reload your rig into maya by replacing your current reference with the new rig and you still lose animation you will have to check your namespace editor. Confirm in one of your old maya saves that the namespace is consistent and if not of course change it to be so. If the lost animation is caused by a namespace issue this should do the trick and restore your work back on to the rig.
ALWAYS KEEP THE OLD RIG VERSIONS
At last it is once again worth mentioning that working on a referenced character is recommended as oppose to importing a character. Should something change in the rig on an update all you will need to do is replace your rig file and refresh your your maya scenes reference editor. If you have imported the character you will have to manually transfer the animation curves from one rig to the other. Referencing is by far the fastest workflow and should something for some reason not work for you in the updated rig you can easily jump back to your previous rig and file us a bug report.