Need a Diagram for push-pull pot, switch btw 500K and 250K pot value

[Jack_TriPpEr]

I have a 500K push pull pot, that i would like to use as a volume pot, and when it is pulled up, engage an extra resistor to change the pot value from 500k to 250k.

I've searched on the web and this forum but couldn't find an exact match for what i'm trying to do.