As described below it could be a case that e.g. a select field had a value type configured as Long, however, no converter specified. The underlying JCR back-end would handle that correctly (would convert select's internal string value to Long and would store it appropriately). 6.2 fields have a similar logic actually (see JcrItemPropertySet implementation), but we still bail out from attempting to convert such cases in AbstractConfiguredFieldDefinitionConverter:

        if (oldDefinition.getConverterClass() == null && oldDefinition.getType() != null && !oldDefinition.getType().equals(PropertyType.TYPENAME_STRING)) {
            if (!supportsType(oldDefinition.getType())) {
                log.error("No converter set for non-String field:" + oldDefinition.getName() + ": " + oldDefinition.getType());
                return false;
            }
            log.debug(oldDefinition.getType() + " type is used in definition " + oldDefinition.getName() + " without proper converterClass.");
        }

Can we just let it slide if the values are primitives?

The example case:

Option group field conversion is failing in dialogs. To see an example, open the pages app and go to the page /travel/contact. Click the main section, scroll down the page to the email field. Edit the field.