Too big

Apparently:

While the new version of Blogger is no longer in beta, some users with certain types of blogs will not immediately be able to switch to it. We'll be adding support for these blogs as soon as possible, so everyone can join in the fun. But for now, if you have a very large blog (more than a couple thousand posts + comments), you'll need to hold off for a bit.

GF guesses that as his blog exceeds 2600 entries, he falls into this category!