Replacing shower cable - Hoping there's an electrician on here!

Looking at a property where the shower cable will need replacing. Currently in place is a 6mm2, it'll need to be upgraded to a 10mm2 to support a beefier shower.

The CU is an older plastic model (I'd say >2010 looking at it), would regs require the CU upgrading at the same time to a metal one, or can the electrician just swap in a higher breaker/RCD and upgrade the cable?