[Keith]

The 1997 logo is still in plenty of places and will likely remain there for another year or two more I'd say.

As the old and new logos are relatively similar if they were to replace them all within a couple months you'd have headlines of "BBC spends £xx thousands to change the font on its signage".

Much better to roll it out slowly over a couple financial years. Also, in 1997 the new logo was a lot more noticeably different to the one it was replacing.