They are trying to nullify RU/CN veto power
That nullify nothing of the veto powers.
This is the current procedure of convening a GA emergency special session:
- Rule 8(b) – Summoning at the request of the Security Council or Members
- Emergency special sessions can be convened by a vote of nine members of the Security Council, or a majority of United Nations Member States. These sessions must be convened within 24 hours of any votes.
- Rule 9(b) – Request by Members
- Allows any Member State of the United Nations to request the to convene an emergency special session.
Rule 9(b) allows any member state (such as Ukraine or US) to call for such session by asking the Secretary General.
The only effect of US proposal is to bypass the secretary general. The proposal does not alter the legal status of the resolution of the GA session which is non-binding.
No resolution, GA or UNSC, can nullify veto power which is in the UN Charter. Any change to the charter has to be firstly agreed by UNSC before being put to GA for discussion. Any attempt to nullify veto power would be vetoed in UNSC.
It is only a "big deal" if you wish it to be big.