the launcher will automatically replace them you need to use a custom ui mod
Heres a link to one:
http://forum.blockland.us/index.php?topic=199196.0
oh boy a link
Just a note if you're using this, you will notice slowdowns when opening some GUI for the first time. This is mainly the brick and avatar menus, but others will do it too.
It's also incompatible with some client mods like TBMI and DrenDran's Advanced Event GUI.
This mod may not actually work for this purpose, however. I don't think I accounted for messages in the script. Try making sure it's in the config/CustomUI/baseUI/CI folder. If it works I'll honestly be surprised, but you'll probably have to use a different method.
My mod is mostly for custom GUI like Blockland Aero, not for replacing things like that.