I looked into this quite a bit as I'll most likely find myself in the same situation shortly. Please correct me if I'm wrong, but this is my understanding...
SNFO applicant qualifications are, I believe, 20/200 correctable to 20/20. SNA applicant qualifications are 20/40, correctable to 20/20, PRK waiverable in both cases(LASIK = disqualification). If you are classified as a PRK candidate, and you meet the pre and post operative PRK checkups, a waiver is obtainable. The waiver, coupled with paperwork, is submitted through your OSO and you will be considered if an aviation slot is available. Contacts are a disqualification for applicants, however it is possible that the waiver program for specific soft contacts is still in effect for current Naval Aviators and Flight Officers. However, in either case the contact waiver wouldn't apply to you because you are not a CURRENT aviator. (as a sidenote, my recruiter said they pulled the contact lense waiver program so I'm not sure on the status of that)
Short answer - Yes, you have to get PRK before you can switch to air. PRK is considered a voluntary surgery and as others mentioned before me, the Marines does not look highly upon it post-signing. It can be done.
Hope this helped, but please confirm my answer. I've been told, "Trust, but verify". Seems like very good advice to me.
-Matt