according to your explanation, i gues we can do it without any downtim.The process must be in the following steps i think
1-create new ifrgp consisting of new 10g interfaces
2-edit the /etc/rc file accoding to the planned new ipconfiguration (remove the ip adress from the old ifgrp , and assign it to newly created ifgrp and then interchange the ifgrp names with each other)
3-perform a takeover from the other controller
4-reboot the controller 1 and afterwards perform giveback
5-then if everyting is OK , do the same on the other controller
am i right ?