我可以去exchange (first choice), 但係我而家想去second choice O個間, 唔知可唔可以改?

NO...coz u hv ur interview b4!
ur interview would be different if u choose the 2nd choice b4..
so u defeated the other candidates who r choosing the same U (1st choice)with u
but it doesn't mean that u can defeat the other candidates in ur 2nd choice's U
so u cannot change
either accept or reject!