By virtue of having been working dayshift for the last 4 years you have an implied term of your contract that you work those hours anyway, irrespective of what an Company Handbook may say (Google 'implied terms of contract' to find out more). The company wants to change this, which they can try and do through consultation.
It is probably a good idea to offer to try it out rather than just saying 'no' - but make sure the terms of the trial are written down - if they won't do that, you write to them and state what the terms are.
This is a good way to play for time. Make sure the written bit states that 'the trial period is x months long after which a joint review will occur to consider the benefits / disadvantages' (or similar words).
As Dot has said, if they are claiming that there is a good business reason for the change, it can be forced upon you, in which case the requirement of the company is that they give you formal notice of the change (the trial doesn't count - it was a trial and hence part of the consultation), which in your case is probably 4 weeks notice (longer if your contract says so). If you are still declining the proposed change, your job is redundant so the company could declare you dismissed on the grounds of redundancy (redundancy is one of the fair reasons for being dismissed or 'fired' - your terminology).