You’d also need a separate type of sign for kph, but one that still adhers to the current standard (white cirle with a red border and a black number). Or put a separate rectangular sign under it with “kph” to denote that the number in the circle is kph rather than mph.

The problem is that you can’t just replace one, you’d need to replace them all on the road due to the repeater signs. It would be confusing to see some say 60 and others 30 on the same stretch of road as some repeater signs have been replaced but others haven’t.

There’d also be the issue of cars understanding the signs with their sign recognition software, and drivers having to switch their digital spedo, as you usually only see one unit rather than both like on older anaologue speedos.