Hi - I've been changing a lot of my passwords lately to make them more secure (thanks lastpass!). One thing I've noticed is that most places when you change your password you get a screen with 3 fields, your old password, a new password, and a confirm.
When you ask lastpass to generate a password it always fills the new password in the first 2 fields, i.e. the old password field and the new, leaving the confirmation blank. So...you have to copy and paste the generated password to the confirm field, and dig up your old password and type it into the old password field.
I've changed at least 20 passwords lately, and this happens on 95% + of sites. If you're going to assume where the new password should go, you'd be safer to assume the 2nd and 3rd fields, not the 1st.