A program to toggle between prefixes

[StephenG1CMZ]

That sounds like it would be very useful to have.

Perhaps some of the code in my Z UNITS program will inspire someone to find a good solution.

That program supports prefixes, but as it stands it's not what you want:
1 - it is a standalone program (so you'd need to retype your number)
2 -- it is unit specific.

So, as it stands, the program should let you convert, say,
3*MEGA V to V (with my program recognising MEGA as a numerical value)
..but will only do the reverse
3 V to MV
If MV is a calculator-supported unit conversion.

But perhaps there is something in there that will help towards a better solution.