To see any of that upside you have to exercise the option. If the company has the common 90-day post-termination exercise window, that means if you want to change jobs you need to come up with cash to exercise what’s supposed to be part of your compensation, and if the FMV is much higher than the strike price, hand a bunch of cash to the IRS too.
Now you’ve put in your time as an employee and reinvested the salary the company gave you right back into the company, and you’re likely at the back of the line with common shares.