At the end of the last merge:

   new/debian ... upload/<version> [tag]

Since last merge (potentially, almost certainly on the Ubuntu release cadence)

   new/debian ... newer debian/sid publishes

And (potentially)

   <version> ... newer publishes to last devel release merged

So the resulting graph looks a bit like (reading from top down, older to newer):


              new/debian
      /                           \
     v                             v
    debian/sid+1                    upload/<version>
     |                               \
     v                                v
    debian/sid+2                       import/<version+1>
     |                                  \
     v                                   v
     ...                                  ...
     |                                     \
     v                                      v
    latest debian/sid publish (debian/sid)   last devel publish (ubuntu/devel)

So now we should be able to "just" follow our process for reconstruct/deconstruct/logical of the ubuntu/devel version and rebase onto debian/sid, *except* that one or more versions are already broken down into their logical parts (upload/ tagged versions).