The Bromley North Branch will now stay 2 466. A derogation has been granted.
This has now been publicly confirmed in Southeastern's December 2019 Timetable Consultation Report. Specifically;
Ensure all of our train services will comply with the Department for Transport’s accessibility requirements from January 1 2020.*
* With the exception of services on the Bromley North shuttle line which have been granted a dispensation.
It looks like the Sheerness-on-sea branch will go over to Class 375/3 operation;
Spread the benefits of modern rolling stock more evenly across the network, including more modern rolling stock on the Sheerness line.
Some mainline services are being lengthened by 1 carriage, which suggests to me that a 375/3 on those is being replaced by a 4 carriage 375 instead, allowing the 375/3 to be cascaded to the Sheerness-on-sea branch.