I think ISP migrations take place at any time. Mine have all occurred during the day, and I've seen reports on here of it happening late afternoon or even during the early evening.
It may depend on what work is needed at the exchange.
An interesting point in the question though. The move from the 20CN DSLAM to a 21CN MSAN is clearly a change at the exchange, and many will be done at the same time. But this also seems to result in the user being placed on a G.DMT, (up to 8Mbps), service in many, (or all?), cases rather than G.993.5, (ADSL2+).
I can see the logic of that, as it saves the package the user is on with their ISP needing to be taken into consideration, but I can't see why the upgrade later to G.993.5 needs any exchange work or BT Wholesale involvement. I would have expected it to be a software setting that the ISP could access at will.
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