http://wonderwinds.com/hackblog.php/2009/05/01/taking-control-of-categories

Basically it's 2 improved files that (a) default the category urlname to 'autogenerate' then (b) inserts your new cat with that urlname and immediately updates the new cat to "cNNN" as the urlname. Like it did when you upgraded to pre-v230 to v230 or beyond. Finally it doesn't get mad when a category urlname is in the format of "cNNN". It'll get mad at m or w or p NNN because I don't know why, but it won't get mad at cNNN.

That means unless you really care about what the category urlname is you don't have to do anything anymore to have one that is acceptable to b2evolution.