Hmm, please tell me you are not trying to hijack someone's Analytics? (I know you are not but your title seems to suggest that?) If you don't have the password there is nothing you can do short of hacking Google itself. I don't recommend it! The best solution would be to use the password recovery procedure. But if you don't recall what email addy was used to set up Analytics or know anyone else who has access and could add another email addy to it, then you can indeed set up a new account and replace the tracking code. I would explicitly let Google know about it though, lest they suspect something fishy is going on. Besides, chances are you will lose the data in the old account (I am not sure about that.)

When in doubt, contact Analytics support!