Sure, please do so (even if it hurts). Modifications are especially allowed and there are only very few restrictions concerning adult content creation or political usage or similar. The restrictions are enumerated in the licensing terms.
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on fair use for both the buyer and the artist/stock company, but I can not rule out that my interpretation is wrong. I'm not an Adobe employee)
Regards, Abambo Hard- and Software Engineer and Photographer.