Coincidence or...

[#1]

My local exchange has been in ‘Red’ (congested paths) status for a long time. It has now just been upgraded. Thank you BTW.
The side-effect may be a coincidence, but…
Before:
SNR Down: 11.7 – 12.6 
SNR Up: 10.0
Output power Down: 11.9 (never varied)
Output power Up: 9.7 (never varied)
Immediately after the upgrade:
SNR Down: 6.4 – 6.7
SNR Up: 10.0
Output power Down: 11.9
Output power Up: 7.7

Attenuation Down (2.5) and Up (3.0) have never varied (the exchange and I are neighbours). Synch speed before and after – 8128 down and 832 up (yip - IPStream Max Premium) – were also unchanged.
But what seems very odd is that the downstream SNR has been more or less constant for ages at around 12 but, after the upgrade, that it suddenly lurched towards the usual Rambo no-interleaving target of 6. But the ADSL line error rates (Superframe, HEC and OCD) have remained low and fairly consistent, and the backhaul ATM error rate almost negligible.
My train of thought goes like this: “if the downstream power is the same and the error rate is the same, how can the downstream SNR be allowed to drop so much – unless, when the exchange is congested, BTW deliberately raise the target downstream SNR in order to automatically trigger a drop in all bRAS profiles which, in turn, will throttle the maximum throughput of most customers off that exchange, share out the pain and damp down lumpy service”.
There could of course be some more prosaic explanation!

[#2]

A coincidence.
The Virtual Paths (VP) referred to in the exchange checker are not related to the DSLAM your line (and thus router) are connected to but the data paths out of the exchange to the BT network. So a VP being red would have no effect on your line condition or sync rate. In fact the VP may not even be the one your data was passing through.
BT network diagram.
SNR values can vary with weather and other interference. That is the most likely cause.

[#3]

So a VP being red would have no effect on your line condition or sync rate.
Quite so. But Rambo via the DSLAM should be aware that one or more paths are congested, and one way BTW could dampen down peaks would be to throttle all users, and a very convenient way to do this without manually changing a whole pile of bRAS profiles would be to increase the target margin...
Having monitored my downstream SNR every day at the same time for the last six months, suddenly finding a 50% reduction immediately the exchange went to Green seems...unusual...

[#4]

But that would only work if they disconnected all the users on that VP to reduce their speed. Not likely.
The VP congestion will in itself limit the data throughput possible, RAMBO would have no need to adjust any lines.