I'm in a similar situation.
I asked customer services regarding the bonus and additional users etc. I was told that to get the 10k bonus, you need to pay the renewal fee and then upgrade when the points post in your account. The plat fee will then be £310 (£450-140) for the year it's calculated automatically but a phone call will sort it in the event of any issues. The fee will then revert back to £450 come plat renewal time next year