>>I mean, if I'm holding a few grand in current $25 and $100 chips, do I risk them becoming worthless when a redemption notice is given?

In Nevada that is correct, the redemption notice marks a period of time (120 days) to cash in the chips.

In New Jersey there is no such process and chips can always be cashed in. People have posted that you can still cash in Playboy casino chips even though the casino is long gone.