Just go ahead and replace it as youre not altering any circuit at all. As an added suggestion whilst youre doing it, its worth going through the screw connections on the shower isolator switch and the mcb (or fuse) terminal just to double check they are still nice and tight, not worn loose.
Also make sure its around the same kW rating just to be sure youre not sticking a 9.5kW shower on a 6mm cable as this would be a bit much.
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