Ali Larijani, who quit on Saturday, will travel with replacement to Rome, Iran says.
Ali Larijani, who quit as Iran's chief nuclear negotiator, and his replacement will go to talks on Tuesday with the EU's Javier Solana on Tehran's standoff with the West.
Analysts said Larijani's resignation, announced on Saturday, exposed a rift with President Mahmoud Ahmadinejad about how to fend off pressure from the West which accuses the Islamic Republic of seeking atomic bombs. Tehran denies this.
The final say in nuclear and other policy lies with Supreme Leader Ayatollah Ali Khamenei. Analysts said changing such a key post would need his approval and Saeed Jalili's appointment indicated support for the president and his tough position.
Diplomats who have met Jalili say he has a reputation for sticking strongly to his views, leaving little room for discussion. In recent weeks, he travelled to Europe to hold talks on Iran's nuclear file.
One diplomat said Jalili, a presidential ally, "specialises in monologue" not debate.
The talks with Solana, the EU foreign policy chief representing six world powers in attempts to resolve the nuclear standoff, take place in Rome.
Foreign Ministry spokesman Mohammad Ali Hosseini said the reshuffle did not signal a new policy and the pursuit of peaceful nuclear technology was part of Iran's "unchangeable goals".
"The other parties must not misinterpret the resignation. We have stressed this time and again, all Iranian officials have said the same, that the nuclear matter is a national dossier," Hosseini told a news conference.
'Commitment and zeal'
Hosseini said Larijani and Jalili, a deputy foreign minister, would attend the talks but said it was not clear if both would go to future meetings.
Iranian analysts said Larijani's presence could be part of a handover plan or to show he was not quitting under a cloud.
The West is seeking to impose further sanctions on Iran for refusing to halt uranium enrichment, a process that can make fuel for power plants or, if Iran wanted, material for warheads.
"Larijani had used his capabilities to buy time and Iran now Iran needs a new negotiator capable of buying more time," said a political analyst, who asked not to be named.
Major powers have agreed to delay a move against Iran until November to see if Tehran cooperates with the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), to answer queries about Tehran's intentions, and to await a report from Solana.
Commenting on the resignation, the English-language newspaper Iran Daily wrote: "There will hardly be any shift in the country's nuclear agenda. However, it is obvious that the new group will pursue Ahmadinejad's nuclear direction with added commitment and zeal."
Ahmadinejad has riled the West with uncompromising speeches vowing no retreat from Iran's nuclear ambitions and declaring the atomic file closed, although he has said Iran is ready to work with the IAEA to allay suspicions.
Larijani is a conservative who has also said Iran will not retreat. Analysts say he has softened his tone during two years as chief atomic negotiator and he has become more pragmatic and been determined to use diplomacy to avert further pressure.
Although Jalili will take over as secretary of the Supreme National Security Council, Larijani will stay on the body which helps formulate policy as one of two representatives of the leader on the security council.
Hassan Rohani, whom Larijani replaced as chief nuclear negotiator in 2005, also still sits on the council as Khamenei's other representative.