Some events allow entrants to switch between distances - usually where they are priced the same - and it's really simple to do.
Please note that all events will set a cut-off date for this sort of change, usually when race numbers are assigned, and once that is passed you will not be able to do this without contacting the organisers.
Log into your profile, go to the My Area menu and select Your orders, you should see something like this (and you may have many orders as opposed to the one that's shown here).
Find the order relating to the event you wish to change and click the Signed up link on the right. We've highlighted it, it's normally just blue! The (1) shows that only one person is signed up on that order.
The detailed order is now shown and you can see the distance that you signed up for. To change this you need to click the Change link under the distance (again, we've added the highlight), If there was more than one person on the order you'll need to select the one that you want to change. NOTE: you can only change one signup at a time!
You can now select the distance that you want to change to from the drop-down and then click the SAVE button.
The revised order details for that signup are now displayed.
Unless you have more changes to make to this order, you're done!