Manager sneakily tried to get me to sign a new position description

I had a meeting with my manager to sign my APR today. Under that document was another document with the final page on top. I started signing it thinking it was part of the APR document then saw where it said "changes to position description" and realised this was a new position description document. No discussion has ever been held about needing to sign a new position description and when asked if any changes had been made they said it was basically the same, just a letterhead change. I told my manager I wasn't comfortable signing something I hadn't read yet and said I wanted to take it away to read.

There are a lot of format changes so it is difficult for me to tell if there are any other major changes however there is one addition which I am worried about as it now states an essential part of my role is to undertake shift work which I do not want to do.

In talking to my colleagues this has very recently also happened to multiple of them but they all felt flustered and signed it without reading it.

I have reached out to our union, but I'm not sure how long it will take them to get back to me and my manager has said I need to get this document back to them by Monday.

Does anyone know what rights I have within this process, as I feel this has been handled in a very sneaky and manipulative way. Can I say no to signing this new document as I wasn't consulted about any changes? Can this affect my employment? How can I hold my manager off until I've heard back from my union? Also, for my colleagues are there any steps they can take as they have signed the form, but again were not made aware of any changes, and felt pressured to sign it in the moment.

TIA!