This is just my speculation... offered as opinion, not facts
The 15 oldest 777-300ER (delivered 2005-2010), some of which were originally delivered as 77W configuration (RL recliner C cabin) are now called 77N (with PL lie flat C cabin). Some 77N have already been converted to 77M (wifi + 10 extra seats but still 9 abreast in Y). These planes have already had their C-check (which requires removal of interior) so not likely to have new interior again until they leave BR's fleet. They will eventually be phased out - probably when their lease expires.
Any remaining 77N not yet converted to 77M will likely get the 10-abreast Y cabin during C-check.
The 10 new 777-300ER (delivered 2014-2016) 77A that were delivered new with wifi will stick around with 9 abreast seats until when they go in for C-check maintenance. I'm sure they will be refitted with the 10 abreast Y cabin as they will be with BR for a long time to come.
All the new 777-300ER that are being delivered now and still on order (B-16728 and higher) will be 10 abreast in Y.