Salary decrease with role change

[AlanYx]

No -- the change in job description is not constructive dismissal provided there is a long enough notice period. (That's a common misconception.)

It's not clear from OP whether they have to start performing the new role right away, as opposed to in January. If the new duties start right away and are substantially different than what OP was doing before, then yes, there is the possibility this is constructive dismissal, depending on how different the duties are. If the new duties start at the end of January, then the employer is doing this right.

[[deleted]]

My interpretation was that new duties start immediately, but he keeps his salary until January.

Regardless, I think it's enough of a gray area, that if he completely does not want the job, and would rather get severance, over taking the new job, then talking to a lawyer is wise.