I do not see the point of transferring. UCLA is already one of the very top universities in the US. Transferring to any school that is higher ranked will be very difficult – in most cases the top ranked universities accept very few transfer students.
Yes, your senior grades from high school will matter if you try to transfer. However, if you do very well at UCLA and graduate with a bachelor’s from UCLA and apply to medical schools (or graduate schools), they will not look at your high school grades at all, they will not even ask nor want to see a high school transcript.
“How hard is it to be accepted as a transfer”
Significantly harder than getting accepted out of high school.
“Assuming I do very well at UCLA (4.0), do I have a shot at any of these schools?”
If you get a 4.0 over four years at UCLA in premed classes, you will be well on your way to a very good medical school. Of course the MCAT will also matter, but UCLA and a LOT of hard work can help you prepare for the MCAT.