Would one be allowed to go to a doctors appointment on chol hamoed (i.e. dermatologist) ? Also would it make a difference if it has already been scheduled before chol hamoed?

ב"ה Shalom , The Shulchan Aruch writes that medical needs may be done on Chol Hamoe'd. ()שולחן ערוך (תקל"ב :ב) The Mishna Brura adds that medical needs include also that which does not involve Piku'ach Nefesh= danger to life. (See also Shemirat Shabbat Kehilchata vol. II, pg. 354. ) Periodical examinations which are not urgent, should be postponed to after Chol Hamaoe'd. (Shulchan Shlomo Vol. II, pg. 147). However, an appointment by a specialist, such as a dermatologist, in your case for which postponement can cause unnecessary suffering, can be done on Chol Hamo'ed. . (פניני הלכה , מועדים ,פני השולחן הלכות חול המועד , עמוד ע) Mo'adim Lesimcha-מועדים לשמחה