Although I don't practice primarily in this field, I do have some comments for you, which I hope will be helpful. Because of the term redundancy, I assume your question involves British law? Assuming this:
1) the first place to look for any redundancy procedure would be in the employees' contracts.
2) even if the employees' contracts do not set out a redundancy procedure, the law provides statutory redundancy rights.
3) because a job title doesn't exist any more does not necessarily mean that anyone with that job title is per se redundant. If the skill sets and requirements of the job are substantially the same, on the face of it, that would argue against redundancy.
I do not have sufficient information about English NHS Trusts to answer whether it is legal to downgrade medical secretaries and re-write their job descriptions and rename the post, but I do wonder whether they would actually BE downgraded in responsibility, salary or benefits.
I wish you luck in resolving this issue.