A:
First, check your union's bylaws and any policies.
They have precedence over RONR.
If your bylaws and policies do not describe the proper procedure, then excluding technicalities, I'd say what you relate may have been within proper procedure, although I doubt it.
I'll assume a quorum was present at both sessions.
I view the essence of the events as:
I'll explain my doubt.
The motion To Amend Something Previously Adopted requires either a two-thirds vote, a majority of those present if notice was given, or a majority of the entire membership.
No adequate notice could have been given, and it's unlikely a two-thirds vote was called for.
Therefore, the deciding question is whether a majority of the entire membership (present or not) voted for the amendment.
A rule of thumb is that a motion rescinding or changing something the organization has already decided requires a more stringent procedure than that required to adopt the original motion.
So, for example, if the original motion required just a majority vote for adoption, it takes more than just a majority vote (such as previous notice or a two-thirds vote) to amend or rescind it.