50.1. When a taxon at the rank of species or below is transferred from the non-hybrid category to the hybrid category of the same rank (Art. H.10.2), or vice versa, the author citation remains unchanged but may be followed by an indication in parentheses of the original category.
Ex. 1. Stachys ambigua Sm. (1809) was published as the name of a species. If regarded as applying to a hybrid, it may be cited as Stachys xambigua Sm. (pro sp.).
Ex. 2. The binary name Salix xglaucops Andersson
(1868) was published as the name of a hybrid. Later, Rydberg (in
Bull. New York Bot. Gard. 1: 270. 1899) considered the taxon to
be a species. If this view is accepted, the name may be cited
as Salix glaucops Andersson (pro hybr.).