Andres;
To confirm and also answer your question...if you hold the metric face aligned against the imperial face, and 100km/hr lines up with 62mi/hr (etc.), it means that the amount the speedometer barrel turns to indicate either is the same, and I expect it will be, so if you have your heart set on indicating in metric, you could indeed "simply" remove the instrument, and replace that face with the metric one, and reinstall...a lot of work IMO!...I'd probably simply add new metric numbers on the existing face with some dry transfer lettering (such as this:
https://www.etsy.com/market/dry_transfer_lettering )
Cheers