Herr BOAC,
Changes to cabling infrastructure are instantaneous for fairly obvious reasons.
What you are potentially seing is the "rate-adaptive" aspect of a BT Wholesale based DSL service kicking in.
That does rely on exchange side equipment do fancy things, than that can take some time (from memory I seem to recall a figure of in the order of days before it settles down and decides on a speed).
For "interleaved path FEC correction", try google....
.... to help you on your way, search results work a peek at may include
Security Forums :: View topic - router stats what do these things mean? and
noisy line - slow speed :: Technical Issues :: think broadband