Hello @Brendan_Woithe
Unfortunately with the way you set up your Exchange Rates Table you will need to use a switch() formula as @Ander showed.
Though there is another way, you can change the Exchage Rates table so it has just 3 columns: Date, Currency, Exchange.
That way you would only have to make a filter of the currency and the date to get the exchange rate with the next formula:
[New Exchange Rates].Filter(Currency=thisRow.Currency and Date<=thisRow.[Date of ER]).Last().Exchange
With this you can get any currency you add without having to change the formula, contrary to the switch formula.