[tex-live] in the future, you can modify the update model?

[Karl Berry]

Oh, I understand now. Yes, you are not the first person to suggest
something like this.

I haven't tried it, but I think our unofficial no-warranty-provided
upgrade procedure would also work to go from an installed tree to the
beginning of the pretest. Essentially, copy the tree, update tlmgr by
hand, then update everything else.

The split between binaries and packages isn't what matters.  The updates
have to be done based on the revision numbers, there is no other way.

For one thing, because we import new Catalogue information for
everything at the beginning of each cycle, many packages will need to be
updated. But I imagine it would still save some download time to start
with a copy of the tree at your end instead of starting from scratch.
--best, karl.