Hi Alex, not sure how to do exactly but maybe worth taking it to a specialist just in case you get it wrong and mess up the system. Usually, if you can do a comparison with the new one, e.g. seeing where the bolts are, etc then that may make it a little easier, but not knowing whether it needs to be removed in the open or closed position may result in extra hassle.
If you are in Christchurch, Dorset then there is a really good automotive trimmer, Kevin Baggs in Sharp Road, Wallisdown http://www.kbaggstrimming.co.uk/ I'm sure he wouldn't charge much just to swap it over.
Let us know how you get on with it
Cheers Trevor