Turkish Airlines Business Class Upgrade Issue

I purchased round-trip tickets for two people from Istanbul to Barcelona and back from Turkish Airlines for the dates 12-15.10.2023. Using 60,000 miles, I upgraded our tickets to business class. About 20-25 days later, I received a message on the mobile app indicating a fare change, allowing for free cancellation/changes or approval of the new flight. I approved the new flight, but upon confirmation, I noticed that our tickets were downgraded back to economy class. I contacted customer service multiple times, requesting to reinstate my business class upgrade. However, due to an error in their application and lack of availability in the upgrade class (J class), my upgrade was not reinstated. They claimed this was in accordance with their rules and policies. It seems that under the Miles&Smiles program, free upgrade rights are minimally utilized, and passengers' earned upgrade rights are taken away under the guise of fare changes to promote paid business class tickets. I am writing this complaint to raise awareness among Turkish Airlines passengers.