Well, what reasons did they have for doing this?
Marketo will keep the original field, but it won't be able to receive updates. Thus, you will have to update any smart lists or Segmentations.
Since Market Segment is a field that is special to your instance, I can't offer any other advice except to go to them and find out
-Why did this change happen?
-Why weren't you consulted?
-Can they turn it back on for a few days while everyone decides on a fix?
-Can we setup a governance model to ensure that changes on either side are properly handled? I'm sure they'd hate it if you decided to start using new picklist values in Marketo.