It's not possible to upgrade to a term with another currency than the one that the user is currently subscribed to. So a user is not able to upgrade, from a term in USD to a term in EUR.
This is due to the resulting proration and currency conversion, where the conversion rate would be subject to daily changes.
In addition, most payment providers process different currencies under different merchant IDs, which would cause problems when attempting to charge the card associated with the subscription, refunds, etc.