Applying for medicine through an internal transfer scheme. How should PS be like?

I'm doing biomed at uni that allows some students to internally transfer to medicine after first year. The process is a bit like how I would apply for undergraduate course after year 12. I did previously apply for medicine but didn't get in then ended up taking biomed since I didn't want another gap year.
So now I have to write a PS. How should I approach this? Should it be like how I would write it as if I was applying as an undergrad applicant or include why I want to transfer from biomed. If so, do I have to say why I choose biomed in the first place, because the fact is I was a medicine reject applicant and it was my fifth choice. but does that not give a lack of interest to the course I'm currently doing, and the fact that the application will first be examined by the biomed department put me in a bad position for it?

Although I have already started the application, just need some advice and hints for this specific situation.