Hi again! Well for me it was a slightly complicated process. My urge of switching courses/universities started in early December of my first year. During Christmas break, I wasted no time and started another UCAS application and actually written a new personal statement as well as informed my old school that I am reapplying again. But at that time, I haven't actually dropped out of Dentistry yet.
I did all of that to basically save time from taking a gap year after Dentistry. By reapplying through UCAS earlier, I had the chance of starting my new course right after the summer break after completing Dentistry first year.
To answer your question of whether or not you should write on your UCAS application that you dropped out is entirely up to you I feel. For me, I was completely honest when I did my application. I wrote on my personal statement that I left Dentistry (note: I haven't actually dropped out at the time yet...haha) because I wish to keep my career options out there more open and that Dentistry is too specialised of a course. Besides that, I wrote that during Dentistry, I realised that I was interested in other things (e.g. biochemistry, economics, cell biology) and wish to pursue that in the future. I reassured the universities that I am willing to work hard the next time around.
Only two of the five universities actually asked for my first year grades. I reluctantly gave it to them because my results were so average, like less than 60%
Thankfully, Imperial accepted my transcript and gave me an offer. I was pleasantly surprised at that. Overall, I got 4/5 acceptances even when I noted on my personal statement I quitted Dentistry.
There is no need for you to tell them. It was just a personally choice for me. They might be curious about what you did during the (gap) year (aka your first year of dentistry). But I guess you could say that you did something else.
Anyways I just realised that the deadline for 2014 entry is due on the 15th of January next year for most courses. If you want to submit a new app for UCAS, you really need to get working on your new personal statement ASAP. But it really depends on whether or not you want to stay in the same university or not. Do you want to switch to another university? If not, you could just ask you tutor on whether or not you can transfer to another course at the same university. It's up to you.