I know I can't, so I take it that people can't switch.
I don't believe you can 'switch' either. My friend looks back on her relationship as a mistake, and thinks back on her feelings at the time that she was bisexual as another mistake she made. In all, she never was bisexual to begin with, and she does understand why she had the relationship, but that is what I won't discuss on here.
I don't believe anyone 'switches' - but maybe it is possible for someone to discover attraction for the sex they didn't know they could. I know this to have happened before, but with that friend it was simply a case of them finding someone that they fell in love with, it wasn't about swtiching at all - but realisation. [this is a completely different friend to the one who had a year-long lesbian relationship]
I just think accept attraction the way it is and accept yourself for who you are.
I completely agree :)