Anybody bought a xONEx fare with an infant before?

Here is an extract from the Rule Sheet:
"Infant: under 2 without a seat – charge 10 percent of fare.
Ticketing code - base fare code plus IN
NOTE – If an infant reaches two years of age after travel has commenced but before travel is complete, a full child fare ticket must be purchased for the entire journey."

This makes an xONEx fare unusual... most fares if the infant turns 2 during the trip, you phone the airline and they provide a seat and there is no additional charge. My reading of the above fare rule indicates that you either have to complete the trip before 2 year old or buy a child ticket for the whole trip.

Is that the way it actually works?