Hi,
Welcome to the forum!
While BT consumer may be known for giving quite typical and canned responses (like many ISPs), in this case they are actually right.
The default SNR (or "noise margin") is 6 dB when on a new provision. In this case it also applies when changing your package as it will reset your Dynamic Line Management (DLM). In order to have achieved 75 Mbps, you were probably on a 3 dB SNR.
There is no set period of time before the DLM sets your line downstream to anything lower than 6 (either 5, 4 or 3 dB). If memory serves me right, anything lower than 6 dB will not be applied unless G.INP is active (I could be wrong, so someone please correct if necessary).
Refrain from rebooting the router too often or making any changes which will make the DLM think you have an unstable line (as this may prevent the DLM from adjusting your parameters). You can learn more about the DLM thresholds here:
http://www.kitz.co.uk/adsl/DLM.htm.
I think the best thing you can do is leave it alone and, as long as the DLM sees your line stable, it should eventually get there like it did before.
Just out of interest, what router has BT provided you with when you went back to them? And which router were you using before you switched to Sky?