Medicine or Accounts and Finance

hey guys,
recently we had to write our personal statements about the course we want to study in uni in 2017 however I wanted to study medicine but due to my bad GCSE'S and low AS predictions I don't think I can get accepted so my alternative course is accounts and finance so if I do get accepted in the universities I will apply to for accounts and finance if next year I get a high a level results and if I score really good on my UKCAT and want to do medicine instead of accounts and finance what will I need to do. I know I can do adjustment through UCAS but what if I don't secure a place as I can only accept one offer in limited time.