Hi Daniel,
Welcome to the Prismic community,
If I understood correctly, you need to handle preformatted and formated HTML differently, and as you know, Prismic writing room (UI) allows to select either preformatted or normal text such as:
And to add a custom behavior/transformation when you output HTML; you would need to use an HTML serializer
Note that we have HTML serializer examples for each technology, so please check our documentation.
Please let me know if you have any other inquires,
Fares