Export and Delete Original

To recover disk space by exporting and pasting over the original file:

Select the presets that you want to keep and click on Export.
Navigate to the presets folder where the original file is located. Name the file the same as the original file but add a number to it. Click on Save.
Quickly select another category, then switch back to the category you were just in. This refreshes the sub-category listing.
Select the original sub-category and click on the minus sign at the bottom of the listing. Confirm on Delete.

The original sub-category will be gone and the new one left in the listing.
Since Bryce exports presets backwards, you may want to re-export the new file, so select all the presets in the new sub-category, click on Export, and give the file the same name as the original file. Click on Save.
Change categories, then change back to the one you're working with. Both versions of the new sub-category will be listed.
Click on the first exported sub-category and click on the minus sign at the bottom of the listing. Confirm on Delete.
Refresh the listing by switching categories and the new sub-category will be listed.