Scotland's so-called other national drink will be produced with less than 50 per cent of its regular sugar content from later this month.
Fans are reportedly stockpiling the drink ahead of the change and a petition has been launched to save the traditional recipe.
Ryan Allen, who started the Hands Off Our Irn Bru campaign, said: "It is a national treasure in Scotland and really is part of our culture with its unique taste, branding and marketing.
"It's also well known to alleviate the effects of a hangover and is many a persons' craving, saviour or go-to drink after a night on the tiles.
"Don't do it Barr - please have a rethink on this."
Irn-Bru's "secret recipe" was introduced by AG Barr in 1901.
The company has been grappling with a shift in consumer tastes towards low-sugar drinks and has been preparing for the implementation of a sugar tax in 2018.
The government levy, due to be introduced in April, is aimed at tackling soaring obesity rates.
Changes to Irn-Bru's sugar content were announced in March last year and will see the amount per can reduce from 8.5 teaspoons to four, going from just under 140 calories to around 65 calories.
The drinks giant has tried to allay fears over the change, claiming their tests show most people can't tell the difference.
An AG Barr spokesman said: "From January 2018 Irn-Bru will continue to be made using the same secret Irn-Bru flavour essence, but with less sugar.
"The vast majority of our drinkers want to consume less sugar so that's what we're now offering.
"We know that our loyal drinkers love Irn-Bru for its unique great taste and we've worked hard to deliver this.
"We ran lots of taste tests that showed most people can't tell the difference - nine out of 10 regular Irn-Bru drinkers told us we had a good or excellent taste match.
"Most shoppers can expect to buy Irn-Bru with reduced sugar from later this month. Give it a try when it comes out."