Pretty sure first tag remains unchangeable and becomes the 'category' choice.
I used to play with my tags too, knowing that they were never used so much to locate content, but nowadays it is tempting to go for the default content tags - particularly as I have stake in all of them.
I like what sports have done with their sub-tags, although had nothing to offer there today, I can usually find one. At present though, I can go through the 'New' link on most tribes - neoxian and marlians are a bit an effort though.