Shinkansen tickets – can I change our reserved seats?

Hi all, I’ve just booked bullet train tickets for my husband and I from Tokyo to Kyoto in May. I was using chat gpt to guide me through (my first mistake!) and when I couldn’t see anywhere to reserve seats, chat gpt told me that I would get the option to reserve seats after payment. I didn’t.

I’ve just realised that it’s about 3am in Japan and I have made the classic mistake of booking when the reservations are down. Has anyone done this before and been able to change their seat?

I’m so annoyed with myself as I love to be organised and I was booking as early as possible to get us decent seats.

I also wasn’t given an option to add our suica card numbers to each ticket – mentioning this in case it matters.

Thank you in advance!