not much, people were already using legacy on the dp and already using it on these, themers will actually care about N, that's about it.
Oms was aosp master and was already set up for N so it isn't gonna effect oms at all (the oms intergrate commit (squash) has a few extras from aosp master for rro, idmap and such for function so they might be stripped since they were merged on master and might be on N) and will be on N within a few days if not the first day.
Most layers exposing commits for hard coded resources are probably still valid if not minorly different and will go to gerrit first for review.
Any need for the conversion framework can be done if needed since that is our compatibility part instead of having it ROM side for themes to be good on mm and n.
The major parts of the to do list can be worked on since we were waiting to see what N brought.
So in tl:dr fashion, N just brings a cleanup to ROM commits and a bit of work but as always layers/substratum works out the box on a new version and back to full running in a week or two.