There is a mapping from old to new Max tariffs.
http://aaisp.net.uk/broadband-transitional.html
The new tariff is cheaper and in some cases has more peak bandwidth allowed.

We are moving around 6% of customers to the new link which should be this month (i.e. before the next bill). We have the ADSL2 ex-triallists on the new link and are moving a small batch of customers next week to confirm it runs smoothly.

We are then moving customers as and when their exchange is upgraded. This means these customers also have ADSL2+ available which is faster for many lines [ED: up to 24Mbps]. This should move reasonably quickly over the next few months.

Soon, but depending on BT, we expect to be able to connect all existing customers lines to the new link to BT and change tariff at that point. This will affect all Max customers. We are hoping some time around November, but this may not be for a few months after that depending on BT. BT's plans have slipped rather. We'll keep you all posted.

Customers on very old tariffs, not Max, are not changing tariff or service until they select a usage based tariff.