hi! i'm doing hoa at ucl
you can try and change but theres only like 4 people doing the joint in my year so even if you have the grades it may be next to impossible to switch whilst there
contact the philosophy department and ask the department tutor or undergrad ad. tutor bc you may be able to change it now as its so far in advance?? i have a friend who changed to philosophy ba from history and said the best thing to do with that dep is to keep on asking a different person until they say you can transfer. that may work better when you have your grades tho.
i was going to change to straight anthropology but changed my mind last minute, but i was offered a place. once you have the grades and they have space its p straightforward.
IF IT ALL GOES TO HELL DONT REAPPLY esp if you like UCL theres no point taking that risk plus sticking with the single hons may work out??
i know people studying the joint and the first year is quite dull you have to do all core modules etc etc which for a year seems kinda painful. single hons actually is much broader than a joint, weirdly enough. if you do the single hons you have to do a subsidiary anyway (1.0unit from philosophy, anthropology, history, or archeology) which you carry on to second year and even to your last year if you want, plus you do a language for your first, and a hoa thematic seminar, both 0.5. so worst case scenario if youre set on philosophy you can still do it with the single hons in a lot of detail
however if you know 100% you want to change contact philosophy and see what can be done and dont leave it till you're here
this is very long but i wanted to let you know the ins and outs! good luck