Op, ask nicely first and then if the HT comes back with a demand for you to attend the parents evening, then you can say something like:
‘Thank you for your fast response, however, just to reiterate, I am unable to attend the rearranged parents evening on that particular date.
As I explained, I have already made plans for that evening that unfortunately cannot be rearranged.
I made these plans in good faith, back in November, after checking the original school calendar for 2022-2023.
I have since also checked with my union, who have informed me that I am not obligated to attend the new date, as no consultation of this date change has occurred and the original calendar was set to protect my work life balance and allow me to make such appointments.
I want to reiterate however, that I do intend to mitigate any inconvenience this will cause, by ensuring I contact all parents in the lead up to the parents evening, and I will endeavour to feedback to my line manager in a timely manner. Alternatively, I am happy to make myself available to meet parents on the following dates:…’
Just be polite, straightforward and leave no room for misinterpretation that you are asking for permission. Equally, give your regional rep a ring, to get advice in writing via email.