Fixed to Max is now free - they dropped that last year to try and encourage ISPs to transfer all their accounts to Max.
AFAIK all other fees still stand including Max to Max Premium
See BT Price list Section 44
Sub part 6.1
eg
Re-grade to BT IPstream Home 500 From - Operative Date Single Payment Exc VAT £
BT IPstream Max Premium
BT IPstream Max
BT IPstream Office Products 30.06.2006 5.00
Which is (one of) the supposed reason why some ISPs dont like putting their customers back on a fixed rate product.
The other being the higher port rental fee for fixed rate (£91.56 ex VAT) compared to Max (£84.12 ex VAT)