Yes, you *can* but I'd think carefully about:
a) Can you shape the current PhD to be what you want to do? If not, why not? If it's that is has restrictive funding requirements - and you're happy to give these up - it would be a good reason. If it's something more nebulous (don't *think* prof would let you do it), be careful these are certainties rather than assumptions before proceeding.
b) Accept that a PhD is often monotonous, repetitive, hard work. This is not a 'bad' PhD, it's often the nature of research. Switching topic will not magically fix this - you'll just realise the thing you thought was interesting is also monotonous, repetitive, hard work to investigate rigorously. Of course, this is a cynical perspective - the ideal situation is you're down the pub on a Friday explaining why what sounds as boring as hell to everyone else is super-interesting - because you're finding the monotonous, repetitive, hard work compelling.
c) It would be up to you, I'd presume, whether you disclose that you're currently doing a PhD elsewhere. As a supervisor I'd rather someone be up-front and explain why they want to switch - and be trying to assess if a) or b) above apply. I'd try to answer these questions to yourself, convincingly, so you can be up front and make the same convincing argument on the application, or rethink things.