if you must change only the default gateway, you don't need to change any config file related to MXOne Appliance. You only need to change it in Yast.
Otherwise you can do it from command line for just in time config:
route del default gw xxx.xxx.xxx.xxx dev eth0
route add default gw yyy.yyy.yyy.yyy dev eth0
and than you can make it permanent by the file /etc/sysconfig/network-scripts/ifcfg-eth0 adding (or editing if exist) the line GATEWAY=xxx.xxx.xxx.xxx
(xxx.xxx.xxx.xxx is the old gateway and yyy.yyy.yyy.yyy is the new one).
I hope is usefull.
Where MML commands can't, linux can. MXONE power.