..this is quite a good site..(...although there seem to be many sites..)...
https://beta.acas.org.uk/changing-an...yment-contract
...although some contracts have flexibility clauses in them, it still has to be changes that are agreed by both the employer and the employee and also with advice from a Trade Union if applicable..(..and without any threat of job loss, that doesn’t form any part of a mutual agreement..)..it has to be shown to be reasoned and not just because larger profits are being looked for etc...I do think that Unions have to step up to these things because the legalities I think, are dubious...