The reason the new syntax is better is a little obscure. Basically, it allows server aliases that contain partial path information. For example, the alias "Dan" could be registered as "byond://dantom/dan" and URLs could be built from this such as "byond://Dan/Impy". This could be done with the old syntax, but then it would not have been possible to continue forwarding topic references to hosted world. (And it just happens that Impy makes use of that feature.)
In case anybody is wondering, the URL path information is passed to client.Topic() just as if it were after a '#' in the URL. The only difference between the two is how they combine when partial addresses are added together. You don't really care about any of this. All you need to know is to use a '/' instead of a '#' when writing URLs to hosted worlds from now on. Enough said.