I am a practicing psychiatrist for 15 years and will switch with you! Just kidding. You should have no problem switching to psychiatry but if you wanted to switch from psychiatry to Derm it might not happen.
Seriously, the pay scale is significantly lower in psych. I believe that all medical specialties can and are practicing psychiatry. They counsel and guide every day. As for learning the medication, that's not something that takes long at all. If you stay in dermatology you can still "do psych."