for what my two cents is worth delta announced MONTHS ago that they would no longer be servicing the cvg/ams route. I specifically remember a "fall/winter capacity reduction" bulletin, but when i went to go find it, the only one i could find was the same bulletin it had just been updated this month so i couldn't find the original date on it. the new items were highlighted in red, but the cvg/ams was in black meaning it had not been part of the update, so for this flight to have been "suddenly" canceled recently would have been pretty impossible, it's just the airline doing a capicty reduction/ schedule change on the flight. alternate options are dtw/ams or atl/ams on NW or DL
as far as your parents not being notified if they booked with a travel agency the agency was notified and was their responsibility to notify you, as when they put your phone number in the reservation they put it in a different area of the reservation and the delta system cannot read the phone number unless it is in a specific area of the reservation. (i'm not saying they did book through a travel agency but offering insight as to why they may not have been notified) also if you provided an email address delta will email you about a schedule change rather than call.
calls are normally placed 6 weeks prior to departure on a schedule change so it may have been you noticed it prior to delta calling you. alot of people catch it prior to being notified by the airline
unfortunately the airline will not upgrade you to first class because of a schedule change or give you any compensation. the only option is a refund but only if you are not arriving within 90 minutes of your originally scheduled arrival.
sorry i don't make the rules at delta i'm just here to hopefully help you understand what is/has happened
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