Won't that arch change mean that old binaries are incompatible?
(I mean, I suppose that's half the point - if you want to verify that your system no longer has any software that suddenly breaks at a specific time, break it all now, but that strategy could also be really annoying.)
Right, thats the point, if you recompile a library with 64-bit-time, then it is ABI-incompatible with the 32-bit-time version of the library, so everything that uses the library would break unless you change the library ABI number and recompile everything that uses it. It would be possible to rename every single library that will be ABI-incompatible, but it would be a lot of work distributed across a lot of individual packages/maintainers and so a rebootstrap with a new system-wide ABI under a new arch name might be less work overall, especially when for many things just a recompile fixes it. There will still be a lot of stuff that needs patches though of course.
(I mean, I suppose that's half the point - if you want to verify that your system no longer has any software that suddenly breaks at a specific time, break it all now, but that strategy could also be really annoying.)