EA bills are converted from one currency to another using OANDA as the source exchange rate provider.
If a direct EA customer changes their Azure Currency Setting, then we will make the necessary conversions using OANDA as the source exchange rate provider. For example: A direct EA customer has an EA bill in USD from Azure and then sets CloudHealth Azure Currency Setting to EUR. The platform would convert cost by looking at the OANDA daily mid-market asking price from USD -> EUR and apply it to each line item.
OANDA historical daily rates and set the asking price to mid-market here: https://fxds-hcc.oanda.com/