The only way to re-register it with a different account would be
resetting it, but if you do that you will lose all your information, and
you won’t be allowed to re-install any application you have paid for,
since they are associated to the account used to register the phone.
In case you want to do that, here’s the official guide:
Reset my phone
However, you could try going to www.outlook.com, and clicking on the “can’t access your account?” link. It will give you some chances to re-gain access to your account without needing to reset your phone.
And as a recommendation, to prevent something like that from happening again, consider using some password management tool. On Windows Phone I’ve used “Password Padlock” and it has worked very well so far.