Are there any issues with changing the default currency after data has been loaded?

I think that it only tells OS which currency is the pivot for triangulation during translation.
Yeah, I guess that's what I was getting at. You need either to or from the app currency in order for OS to know how to translate. So for example, you have the following rates on a CAD app:
CAD -> USD
CAD -> EUR

OS would know how to translate those even though CAD isn't the destination currency. If you were to change the app to EUR, your CAD entities would translate fine but your USD entities wouldn't.