Existing employees can not be forced to take up employee shareholder status, but employers can choose to offer only the employee shareholder status to new joiners.
Shareholders' agreements sometimes include provisions requiring any employee shareholders who cease to be employed by the company to offer their shares for sale to the remaining shareholders.
There will be tax relief too for employer companies offering the new employee shareholder status against the acquisition of shares by new employee shareholders.
Existing employees can not be forced to take up employee shareholder status -- however employers may choose to offer only the employee shareholder status to new joiners.