Dear Magnus,
well, we do not require you to pay again for the software you are using.
Just if you want to receive new updates with new and improved features, you may prolong it. You can of course keep using your current version.
I think the logics behind is clear - we want to keep developing the software for you, but it is a continuos process, so to be able to do so, we need to finance it somehow. Also, the support is quite costly, so I hope you will understand and support further improvements of JoomSEF.
So far, there is no special way for "renewal", we are still looking for the best way how to do it. The simplest way at the moment is to make new order. We can offer you a coupon discount for it.