This 'guide' assumes they're all installed in c:/Nexon/
If you have NA live, KR Live, KR Test and JP Live installed your folder could look like this:
Mabinogi
MabinogiJP
MabinogiKR
MabinogiTest
test uses own registry files thus the folder doesn't need to be renamed at any time and works alongside any Live folder, Live servers however use the same registry thus need the folder renamed or they will overwrite whichever is named "Mabinogi"
During installing a new Live server make sure that all current ones are NOT named "Mabinogi" to ensure them not taking any damage.
Now say you close NA which is currently named "Mabinogi" and want to play KR
Step 1 - Close the Client
Step 2 - go to C:/Nexon
Step 3 - Rename "Mabinogi" to "MabinogiNA"
Step 4 - Rename "MabinogiKR" to "Mabinogi"
Step 5 - double click the Launcher in "Mabinogi"
If this doesn't work and still breaks your NA/KR/JP Client the issue may be with your OS