Because I have seen several posts asking about this, I decided to make a step-by-step guide on how to do it.
Note: Be sure to completely close minecraft before doing anything!
Step 1: Find the save data folders for the world you want to move and the age you want to move it into.
Step 2: Delete everything inside the save folder for the mystcraft age that you want to replace.
Step 3: Move/copy everything in the world save folder that you want to move into the now empty mystcraft age save folder.
Step 4: Fire up minecraft and enjoy your new mystcraft age!
Note: Be sure to completely close minecraft before doing anything!
Step 1: Find the save data folders for the world you want to move and the age you want to move it into.
Step 2: Delete everything inside the save folder for the mystcraft age that you want to replace.
Step 3: Move/copy everything in the world save folder that you want to move into the now empty mystcraft age save folder.
Step 4: Fire up minecraft and enjoy your new mystcraft age!