I have no specific knowledge about 4.9.
But generally, mostly it is about new/changed kernel config symbols and the needed changes to LEDE/Openwrt specific kernel patches (~100 generic, ~30-50 per target platform).
Most of the critical stuff has probably been already done, as the first platforms have been successfully moved to 4.9. If you check the LEDE commit logs, you will notice about 10-20 commits about "changes for 4.9" to several packages.
But there can always be surprises. E.g. 4.4 transition was difficult for ar71xx. Some devices got into a boot loop. After that got fixed, there was a noticeable performance hit due to unaligned memory access. I helped nbd to debug that in May 2016 by trying patches from his staging tree. So, the community can help in the transition by testing the draft patches for platforms being worked upon.