David had to think about this, as a elegant solution isn't that easy ;-)
In the meantime I've found this topic, which requires the same backend functionality to be up and running: https://together.bunq.com/d/12966-sepa-xml-improvements
So, my suggestion is (without having access to the beta and the new design language)
- keep the current type / mapping as default (description -> info)
- (similar to the currency selection, at the amount input screen), add a blue text, which allows to switch between the mappings.
I'd choose a popup like at the refill notifications page, which is used to choose the refill method (Sofort / iDEAL / ...)
Keep the texts really close to the EPC's text, and maybe in a smaller font the next line a description what for this is typically used, the size limitation, as well as allowed characters.
However, @Jakob 's idea would allow to fill both fields for each transaction, but than you'd have to provide a "first-level" distinction simple / expert, and within the expert one, you'd again have to allow a swap between the structured and the unstructured one, with their different length limits.