A lot depends on the combination of line sync speed, attenuation and noise margin, so hard to say much without knowing your old LLU figures and what you have now.
Upstream can be slower on the 21CN WBC network which they have moved you to.
The only time I've not seen older ADSL only hardware work, was on a line with ADSL2+ Annex M which will not be your issue.
It is possible that you've been assigned an IPStream like profile in the switch over, i.e. ADSL only, max sync 8128 and upstream max of 448Kbps. If you can force ADSL2+ on one of the modems, see what happens to the sync rates.
With a 6Meg sync, ADSL2+ will usually add around 1Meg to 2Meg.
It really is more of a cross grade, than an upgrade, the people you talk to will have been told its an upgrade, but reality it is not, for some due to DLM they are seeing slower speeds.