Hi
I don't think there is a tool to change the default tools.
Have a look at "SYS:Prefs/ENV-Archive/SYS/" there you could find all of your default icons.
By left-clicking once on these icons and than selecting the menu-item "Icons/Informations" of the Workbench menu you could change the default tool of every of these icons.
The icons used for the compressed files are named "def_lha.info", "def_lzx.info", "def_gzip.info" etc.
It's a lot of work, cause you have to change all icons that have "SYS:Tools/Unarc" as default tool.
Don't forget to make a reboot after finishing so the changes are copied to "RAM:ENV" (or you might do the same on every icon found there too ;-))
Noster