The managing director of Emap Elan, David Davies, said the only elements of the magazine that would not be changing were the name and the fact that it is targeting women.
"It's a new launch, rather than a relaunch," he said. "It's a completely different product."
Davies said the move, spearheaded by new editor Jane Ennis, was akin to Heat's decision in 2000 to scrap its broad entertainment format and focus purely on celebrity gossip.
"It's that level of change," he said. "The name is about the only thing that won't change. And it remains a women's title. But everything else is completely different."
The title, which has changed its frequency to fortnightly during August and part of September, is likely to focus much more on lifestyle and celebrity, billing itself as a slightly older, less glossy equivalent of Grazia.
One senior press buyer said Emap needed to act quickly to overhaul First. A rival title from H Bauer, In The Know, folded after just eight months in May this year.
"Emap always seem to turn around its flagging titles, but nothing seems to be happening with this," the buyer said. "It doesn't seem to grab the consumers' imagination."
First, which launched in May 2006, posted a debut ABC figure of 100,439 in February, and subsequently dropped its cover price from £1.20 to 99p.
At launch, the title set a first-year circulation target of 150,000 - but it is understood the title has struggled to even break the 100,000 mark in recent weeks.
Last week, a spokesman for Emap said: "First will return to the news-stand with a weekly frequency in September as a key member of the UK's most modern and exciting women's weeklies portfolio, alongside Emap stablemates Closer, Grazia and More."
Emap also recently announced that it would be taking fortnightly More weekly and would pour millions of pounds into repositioning FHM to appeal to an older, more sophisticated audience.