Presumably you are switching to get one of these £100 incentives for switching. I do it regularly but Idon't always close the previous account- i have one account which I keep ticking over and stays with me. The reason is that some credit providers ask how long you've been with your bank and it's better to say 10 years than 10 days. £100 is a good incentive and there's no harm in doing it- the switchover of DDs etc is usually fairly painless