The leader, facing a potentially hostile reception from some trade unionists in the wake of the Falkirk fiasco, will tell the TUC conference in Bournemouth that a new One Nation Labour Party has to be built but, in a sign of rapprochement, will insist the unions are "not the enemy within".
On his plans to end automatic affiliation to the party by three million trade unionists, Mr Miliband will say: "This is a historic opportunity to begin bringing people back into the decisions which affect their lives.
"It means we could become a Labour party not of 200,000 people but 500,000 or many more. A party rooted in every kind of workplace in the country, a party rooted in every community in the country, a genuine living, breathing movement."
Describing it as a "massive challenge", he will add: "Like anything that is hard it is a risk, but the bigger risk is just saying let's do it as we have always done it."
However, an ambition to increase full-time membership by 300,000 means the Labour leader expects as many as 2.7 million currently affiliated members might not sign up; this translates into nine out of 10 affiliated members opting out of voluntary membership.
Backbencher Ian Lavery, who chairs the trade union group of Labour MPs, described Mr Miliband's proposed reforms as "ill-thought-through" and warned the party would face "financial meltdown" if they went ahead.
If other unions followed the lead of the GMB, which plans to cut the amount it gives to Labour in automatic affiliation fees - a move which would cost £1.1m in 2014 alone - the party would be "somewhere in the region of £8m to £9m" worse off, he suggested
"I'm not sure whether the party could sustain such a financial loss," Mr Lavery added.
As conference backed a day of mass action on November 9 in protest at pay restraint, Len McCluskey, the Unite leader at the centre of the Falkirk row, said the controversy had damaged the party in Scotland and that there were still "smearmongers" at large. However, he added that Labour had to move on to win the next General Election.
"I suspect the public are fed up with Falkirk. We have to work to repair the damage," he said.