Separately, IAEA experts are scheduled to visit Tehran Feb. 13 in their more than year-long effort to restart a probe of the weapons allegations.
Iran insists it does not want nuclear arms and argues it has a right to enrich for a civilian nuclear power program. But suspicion persists that the real aim is nuclear weapons, because Iran hid much of its program until it was revealed from the outside more than a decade ago and because of what the IAEA says are indications that it worked secretly on weapons development.
Defying U.N. Security Council demands that it halt enrichment, Iran has instead expanded it. Experts say Tehran already has enough enriched material for several nuclear weapons.
Nonproliferation expert Mark Fitzpatrick described the planned upgrade as a potential "game-changer."
"If thousands of the more efficient machines are introduced, the timeline for being able to produce a weapon's worth of fissile material will significantly shorten," said Fitzpatrick, of the International Institute for Strategic Studies.
"This won't change the several months it would take to make actual weapons out of the fissile material or the two years or more that it would take to be able to mount a nuclear warhead on a missile, so there is no need to start beating the war drums," he said. "But it will certainly escalate concerns."
A Western diplomat accredited to the U.N. agency said IAEA delegation heads from the United States and its allies planned to discuss Iran's plans later in the day. He too demanded anonymity because he was not authorized to talk about the issue.
Iran is smarting under U.N. and other international sanctions for refusing to curb enrichment and physicist Yousaf Butt, a consultant to the Federation of American Scientists, said Iran was "using the only leverage it has — its enrichment program — as a means to coax some sanctions relief."