When changing a fields dictionary type from from String to HTML or vice versa, the formatting is lost on those fields (i.e Description).
Only the raw data is preserved when a field's type is changed. Therefore the formatting of the field's value will be stripped away.
Unfortunately, when a string type field is changed to an HTML type field, you will find that the formatting is lost and only the data will remain. We do not have an out of box solution which will help you retain the desired formatting at this point in time.