Is it possible for a lesbian to switch to men like a bisexual can?

Why not it is their preference. It depends on whether the woman really is "lesbian-only" or whether she's bisexual. Incidentally, the term 'preference' can be misleading. It sounds rather like someone shopping in a supermarket and saying, 'Today I'll really spoil myself and buy the top-o'-the-range bixsexual premium product.' Even if there's an element of choice involved, it doesn't work as simply as that. For example, it may take a really special man to unlock her hetero side if she has one.