I am on Pay and go with o2 and my Tariff is due to renew on 21st nov and i am on Simplicity Unlimited TEXT + 500MB WEB and i have changed that to the normal text and web and thats due to start also on the 21st nov but i would like to know should i top up with the £15 before the new tariff starts and wait till the 21st nov when it changes?

i just dont want to top up before to find out it does not count towards the new tariff.

You need to top-up before your anniversary date. When it gets to your anniversary date, O2 look at the total amount you have topped up since your last date and give you a free allowance based on that.
If you don't top-up until the anniversary date, there will be a short period when you don't have a free allowance.
This is explained more clearly on the O2 website.

Even if im changing Plans?