Are you in a union? Have they had a say in it?
The implication is that the new JD is likely to have some changes compared with what your job currently involves, and in being asked to think about whether it's something you'd want to do, that you're not going to think of these changes as an improvement.
Do you know what the new JD is likely to include? I don't see how you can be expected to have an opinion on it if you don't yet know.
Although you haven't had a formal JD, you have been doing your job for 5 years. There must have been some sort of description when you applied for it? In any case, I would write down my own JD of how my job is now, what I actually do, so that I have a starting point to compare and contrast when the new JD is seen. I'd probably also make note of any changes in the last 5 years - it's likely there will have been some, doing more of X, less of Y, simply as a response to changes in the business in that time. Also, if you have annual objectives or the like, they are an indication of what you are expected to do. Do they fit in with the new JD?
Do you know if this is an attempt to formalise things already in place, or if it's in response to changing requirements, streamlining and efficiency or something?
None of that answers the question about whether they can change your role without consultation (and perhaps that's what they consider telling you you need to think about it to be,) but it gives you a starting point to be able to see how much of a change is being suggested. (Plus, change can be for the better!)