The Public Comment Period closed on February 16, but not all of the submitted comments have been posted yet. The proposed rear underride rule can be seen here along with all of the posted comments: NPRM Upgrade Underide
On the question of whether used trailers should be retrofitted in order to make them safer (better able to protect against deadly underride crashes), here is an opinion from one person who recently submitted a public comment. . .
Decision not to require used trailers to be retrofitted (end of Section 7, page 32):
“You state that your analysis indicates such a retrofitting requirement would be very costly without sufficient safety benefits. If more lives would be saved and more injuries would be prevented by requiring new trailers to meet the new standards, then logic dictates that more lives would be saved and more injuries would be prevented by requiring used trailers to be retrofitted.
“Indeed, the crash tests cited for new trailers of different manufacturers tested on a Chevy Malibu suggest substantial disparities. It can be expected that the disparities would be magnified if tests were conducted on some of the used equipment in the nation’s fleet. If it is not worth the cost to retrofit an old trailer (in which case it should be scrapped), that should be the decision of its owner rather than the decision of NHTSA.” See more of his comments here: Comment from D. J. Young, III
Here is one innovative side/rear guard design which will be tested soon and could be a possibility for retrofit: Innovative combined side & rear guard promises better underride protection.