Maggie Ardito,
Glad to hear it is working well. You might be able to use ArcGIS Online assitant to change your layers over (ArcGIS Online Assistant ). This will let you change the endpoints of your layers so that you don't need to repoint your story map to the new layers.
You can also duplicate your storymap and then repoint all the underlying layers to make sure it is working before deleting the original data.
Glad to hear that it is working for you and that is a huge reduction in size. At 2.4 credits per 10mb of hosted feature layer storage you should be looking at spending only 0.72 credits per month which means you will be spending less per year then you were spending in a month!
Let me know if you need any more help!
Thanks
Ben