It is easy to find a lot of advice online about Agile development, but unfortunately this also means it is easy to find a lot of bad advice online.

For example we have this advice on changing your sprint length:
http://futureofcio.blogspot.co.uk/2014/01/ideal-agile-scrum-sprint-length.html

It is important to be a bit sceptical about any advice you read online about Agile development (including anything you read from me). One of the best points to come out of the “Is TDD dead” debates was from David when he said that Agile and TDD have become very broad tents with different people claiming completely different things are how to do them. Agile especially has become a cliché. But that doesn’t mean there is no value in discussing the pros and cons of different practices. Far from it.

In the case of changing sprint lengths, you can find some good advice here:
http://www.benday.com/2014/07/24/really-bad-idea-change-sprint-length/