If the right to full pay definitely forms part of your existing contract, then your employer can't arbitrarily change it:
https://www.gov.uk/your-employment-contract-how-it-can-be-changed
However the employer might argue that there's actually nothing in your contract giving you such entitlement and that the provision of full sick pay up until now has simply been a perk which can be withdrawn at any time.
As a rough analogy, when I worked on the railways, jobs were advertised with the inducement that employees would get a free travel pass, which they could give to anyone they chose. The company later changed the rule so that only a spouse could receive such a pass. That caused uproar among staff who'd given their free pass to the their son/daughter/brother/sister/mother/daughter/friend/partner etc, who protested that it was a change to their contract. However the legal teams of four different trade unions all looked into it and unanimously agreed that the free travel pass had only ever been a perk and not a contractual entitlement. So you might be in a similar situation, whereby full sick pay has only ever been a perk, rather than part of your contract.
Your employer was obliged to provide you with information about sick pay on the first day of your employment:
https://www.gov.uk/employment-contracts-and-conditions/written-statement-of-employment-particulars
While such a statement isn't, per se, part of your contract, it might still prove to be useful in arguing that you have a contractual right to full sick pay.
Your best source of definitive information will be your trade union's legal adviser or, if you're not in a trade union, the Acas helpline:
https://www.acas.org.uk/contact
Many AB members have used that service, with the general feedback being that it can take quite a while to get through on that number but that it's always been well worth the wait.