Unfortunately there’s no way to accomplish this right now. This scenario has been brought up with the product team on this as something to think about. Until/if we get something figured out, there’s no full solution I can readily think of in the meantime.
If you have a lot of attachment to the numbers, one mitigation could be: For pages that already exist, I’d recommend keeping the original domain1.com addresses active and presented as the URL to the Tweet Button for those older pages – then keep domain1.com redirecting to domain2.com’s representation. For pages authored after the cut-off date, use the appropriate new URL.
If you don’t have a lot of attachement to the old numbers, go “all in” with your new domain and start over from zero, perhaps growing the numbers through innovative new ways you find to promote discovery of your older content all over again…
