My biggest worry personally (aside from the realpolitik of
getting this change through) regards the automated partitioning
language available through the preseed system. Trying to emulate
this bug-for-bug is a non-starter, I think, both from a
technical and quality-of-life standpoint. If the emulation can't
be perfectly accurate, I don't think it ought be attempted for
such a critical, delicate procedure.
I personally think that preseed is nasty enough that users who do automation
on a scale that would make learning it worthwhile already have a better way to
do such automation. For me, d-i is for manual installs, scripted stuff
wants a partitioner + glorified debootstrap.