we would like input on how a transition to new support code
    can be managed.

Forgive the naive question, but is it viable to make the ltluatex code
not actually break loading the existing packages?  I mean, if a document
using the existing packages can go right on using them in the presence
of ltluatex, then all seems well to me.  If that's not possible, it
seems to me that every existing lualatex document will require changing.
Which seems obviously undesirable, if, I suppose, just barely possible
at this point.