pigman wrote: But then there'd be the problem of differing terms & conds for new & existing employees.
A fairly common situation in many workplaces these days - think defined benefits vs defined contribution pension schemes, company car schemes, health care packages, etc.. Many employers choose to deal with changes like these in exactly that way - existing employees get one deal, to be continued ad infinitum whislt the newbies get the change. Unless you work for HAL computers in which case you just get a letter saying "From dd/mm/yy nnnnnn has changed if you don't send back the reply slip saying you agree by dd/mm/yy then you will be considered to have resigned" (and they got away with it.)
My sense is that, from the employer perspective, when such differences benefit the employer they are rarely a problem, when they benefit the employee they generally are. (But then I'm a cynic, a Christian Socialist and a trade unionist so I would say that wouldn't I?)