Talkalot is the base tariff (what you are charged when you make calls/texts etc. outside of your allowance) O2 unlimited is your free allowance, you are given your free allowance once a month on your anniversary date. Until then you will have the free allowance from your old tariff.
The new tariff will be added to your account within 48 hours of the request and will swap over at the next anniversary date. The text message you recieve confirms the change has been added, not necessarily that the new tariff has swapped over and become active. You don't have to top up to 'activate' the new tariff. As long as you have topped up in the last 30 days you will get the appropriate allowance based on how much you have topped up.