Good morning Naomi,
The rider can fill in a horse as Cat 1 in the entry system, and this will be imported as such.
Normally as long as this type of start doesn't withdrawn you would directly notice it. When a category is fully withdrawn, the class will "split" for this category.
You can safely change the category in the start list to H, this will be preserved in your show when you reimport again.
Best regards,
Nick