Hello Alex,
Upgrading only costs the diffrence in price between the two versions, you may upgrade at any time and upgrading also renews your upgrading plan with 1 year from the date of purchase.
We have not been able to add Wordpress 2.8 support, the authenthication method was changed again in Wordpress 2.8 by the Wordpress team and we have not been able to make user integration work with 2.8 yet. We are working on it and a compatibility list will be up soon showing which versions work an which ones are scheduled for implementation.
Kind Regards,
