I would imagine it will be like the old 50Mb and 100Mb services were initially, with DOCSIS 3.1 CPE becoming more commodity hardware as time goes on. Think about the money VM are spending on the new 24 channel CPE. 3.1 renders those redundant. No way they'll be simply consigning every 'Vivid whatever' the customer is on the top tier at the time's SH3 to the scrapheap.
EDIT: I also imagine they will be wanting to bed the technology in and not throw too many punters on it at once. For a glorious but brief period I was the only customer on 3.0 on the entire Mortlake hubsite. Unsurprisingly, pings and jitter were rocking
