Often teachers within secondary schools have not ended up there by mistake. It is usually a conscience decision to go in to secondary school teaching from early on in there training. They may have opted for this due to the love of a particular subject or the have the opportunity to teach the challenging and inquisitive minds of teenagers.
Sometimes though secondary school teachers decide that it is no longer for them and that they would like to try teaching in a primary school. Although you do not need any additional or different qualifications, the two jobs are often quite different and you will need to learn to adapt quickly when switching from one to another.
As a primary school teacher you will need to have a wider breadth of knowledge as you will be teaching all subjects not just one. You will also need to know who to deal with different types of issues to ones you have probably been used to in secondary schools and may have to learn to ease up a little as primary students are of course younger. As a primary school teaching who has moved from a secondary school post, you will also have to contend with moving to being a generalist rather than a specialist, and with having the same class all the time.