Unless there is a leak, the only things they will likely change are filters. Maybe add a little gas if pressure down. and blow out condenser.
If your filters have never been touched, then it's likely dirt has got past, and maybe that's what he means. They do get very grubby here.
If as you say, it's blowing much colder, then they have added gas.
GF's Honda City went in for service last week. They said they think there is a leak somewhere in the A/C. Said they will need to change three parts: Condenser, expansion valve, and I think the other was evaporator.
That says to me they don't know where the leak is and just want to change everything. For over 12,000 baht, they can stick it.