I've always wanted to put this one together but none of my clients (since 2002 with version 1.0) have ever wanted to spend money for it - what does that say about it being a 'nice-to-have' instead of a 'must-have' requirement!!
This automation uses Flow to retrieve the latest currency rates from a public feed (thank you http://www.floatrates.com!!) and update those currencies set up within Dynamics 365. This flow provides the basics but certainly there is opportunity for improvement. Please do!