Changed my bike from 6 to 12 volt. Do I have to change the 6 volt ammeter to 12 volts.
The ammeter is in series with the electrical load and offers virtually no resistance to current flow. There will be no appreciable voltage difference between the 2 terminals, irrespective of the system voltage. On your original 6v system you would measure 6v (nominally) at both ammeter terminals. After changinging to 12v you would measure 12v at both terminals. Assuming the same wattage the current will halve on the 12v system and so the ammeter maximum scale will not be an issue either.
Just noticed that my ammeter has + to - of 30. Is that too large a scale for the needle to move on a 250 Jubilee.