The statement, which was issued amid a conference held by the umbrella HNC group in the Saudi capital, Riyadh, further said that the delegation would be headed by Nasr al-Hariri, a senior member of the so-called National Coalition for Syrian Revolution and Opposition Forces, in the fresh round of the peace negotiations, scheduled to be held on February 20.
According to the text, a total of 21 delegates will take part in negotiations.
No reason was given for the decision to replace either Zoabi or Alloush.
Syria has been locked in a devastating civil war since early 2011, when the Bashar al-Assad regime cracked down on pro-democracy protests - which erupted as part of the Arab Spring uprisings - with unexpected ferocity.
But representatives from both groups denied they were included in the delegation.
De Mistura has even gone as far as to threaten to personally select the opposition delegation for the talks if opponents to the Syrian regime can not agree on a delegation.
The Moscow grouping includes former minister Qadri Jamil, and is close to the Russian leadership, while the Cairo grouping includes former foreign ministry spokesman Jihad Makdissi.
De Mistura had warned the opposition that he would pick their delegates to the talks opening in Geneva on February 20 if they could not decide by Wednesday on who would represent them. De Mistura planned to resume discussions last May but the date of the meeting had been postponed many times.
Russia's foreign ministry signalled on Thursday a second round of talks in Astana to be held on Feb 15 and 16, nearly a week before the UN-sponsored Geneva talks.
The Kurdish ruling Union Democratic Party (PYD) had earlier said that they had received assurances from countries including Russian Federation, that they would attend the meetings in Geneva, a move strongly opposed by the Syrian opposition, and one of their regional backers, Turkey, which considers the Kurdish group an extension of the banned Kurdistan Workers' Party (PKK), and therefore a terrorist organization.