Here's how...
1. SSH into /private/var/Applications/Preferences.app/English.lproj and copy the Sounds.string file to your PC
2. convert the Sounds.string file using
http://140.124.181.188/~khchung/cgi-bin/plutil.cgi
3. open the converted Sounds.string file using WordPad or any other editor
4. change the SMS ringtone names into your preference and save changes
5. copy back the amended Sounds.string file back to your phone (make a copy of your original Sounds.string file, e.g. rename it into Sounds.string.original)
6. done!
All 6 SMS ringtones are located at /System/Library/Audio/UISounds with names from sms-received1.caf to sms-received6.caf [with corresponding name Tri-Tone etc.], Google for ways to create SMS ringtones if you have no idea how to do it...