If you think that you can PROVE that the company is deciding upon who to give the new job to on the grounds of age, then see the links under 'Further information' at the foot of this page:
https://www.equalityhumanrights.com/en/advice-and-guidance/age-discrimination
The Acas helpline can also provide advice:
https://www.acas.org.uk/contact
However getting recognition that you've been discriminated against and being able to do something about it are two very different things. When I was unemployed, I saw a vacancy advertised for a graduate where I thought that there was a near perfect match between my c.v. and the job requirements, so I sent off for the application form. When it arrived, however, I found that the word 'graduate' in the JobCentre ad had become 'recent graduate' in the printed details that accompanied the form. I complained to the
Equality and Human Rights Commission that the employer was, in effect, specifying that they wanted a young person for the job (as the vast majority of older graduates wouldn't meet the 'recent' requirement). The EHRC wrote back that, while they agreed with me in principle and that the employer shouldn't have included the word 'recent', they felt powerless to act formally because the employer could argue that
some older graduates would be 'recent' ones.
To return to your actual question though, the employer wouldn't be able to drop your money without your agreement (or, rather, if they did, you'd be in a good position to claim for effective unfair dismissal). If they were able to show that the old lead role and the new one were two entirely different jobs though, they might quite possibly be in a position to make you redundant. (Use the Acas helpline for advice).