I believe the provided solution works perfectly for the desired use case, maybe needs some testing/adjustments regarding the number.
assume you have a single visit where user searches 3 times for "madrid", twice for "barcelona", that would be "2 different values" for the "city_combo". that means if you are looking for users who made 1 change, that would be 2 "distinct count" for the desired value.
I would take the segment provided by asheep above and do the following modifications:
1) change to "visit" container
2) change to "equals 2" or "equals 3". there might be cases where the user didn't have a value for "city combo", that means the item has a first value of "undefined". so looking for "distinct count = 3" moght be the right condition for those users in question (unspecified/madrid/barcelone).