I think the reason for switch was the night Niki abused Zoe and Ivo did nothing or not enough and too late to stop it.
Now he either feels guilty against them and cant stand to live with them because of that. Second possibility is the vacation was not so voluntarily and VV punished them by temporarily taking them out of the project.
After that night Ivo and Sonia for sure lost their lets they leading position in VV. So Lex and Iris will be the new leading couple now.
I still dont think Ivo is a bad guy but no matter of he was drunk or not his actions on that night were simply wrong and not tolerable.