Noise margin

[#1]

Hi,
Could a shifting noise margin cause disconnection? It ranges from 7db to 10db frequently.

[#2]

Not at that kind of value and range.
Your 'Target' margin is probably 9dB; 10 is the maximum during the day and 7 the minimum overnight.
The nominal Target is 6dB, and a disconnection could occur at various values below that, depending mainly on the individual line characteristics and the model of router.

[#3]

My exchange is apparently getting updated to 21cn next month, I wonder if preparation for that could cause problems?

[#4]

Now that does sound quite feasible.