Unite said five plants at the huge site in Scotland will close, while shift payments will be cut by £2,500 a year and new employees will receive lower wages.
Owners Ineos threatened to close the petrochemical site in October with the loss of over 800 jobs unless workers accepted changes to pay, pensions and terms and conditions.
Unite accepted the changes, most of which are due to come into force in January, but the union said the job losses were new.
Unite's Scottish Secretary, Pat Rafferty said: "The high price Scottish and UK taxpayers, BP, the workforce and the wider community are paying to save Grangemouth is becoming clearer by the day. People with years of loyal service are paying with their jobs and their livelihoods.
"While Ineos' owner Jim Ratcliffe rejoins the billionaire's club, they face a bleak Christmas and New Year thanks to the price being exhorted by Ineos. It's a price that would shame Scrooge and a price which could ultimately be self defeating, leading to an exodus of the workforce and the senseless deterrent of skills from the site.
"Ineos may have their new Ethanol tank and a future at Grangemouth, but everyone else including the local economy is taking a tanking to get it."
No-one from the company was available to comment.