Update:Thanks for putting out the complete steps.
Oxygen Updater shows me only the full update package of 4.41 GB for C65. I don't get any incremental update with this. Whereas the built-in System Update shows me the incremental update of 410 MB which I think is strange because I have Magisk 25.2 and it is working correctly.
Did you mean the full ROM to be downloaded at Step 3?
I also have the 'Stock backup does not exist' message like the other user. I searched for the stock boot image within my phone with a root explorer but am unable to find it. So I will go first with booting with the magisked boot image of C63 and then install Magisk as per OP. Then attempt the update to C65 again.
But after my question about Step 3 is clarified.
I booted C63 magisked boot and then used direct install to install Magisk again. Then the stock boot image was saved. Then I followed @mlord's steps above and downloaded the full C65 ROM (since I had the Stable (full) enabled in the Oxygen Updater). Installed the ROM and installed Magisk to the inactive slot and booted. Now I have C65 with root.
Thanks again @mlord!