OK. I'll jump in here. Clearly I don't know you or your employer and I don't have a full insight into you or your health; diabetes or otherwise, so I may be concentrating on a tangent.
.I would suggest you may have brought this upon yourself by pressing to have your shifts changed now; citing your diabetes as the reason for that. From what I read, you objected to the delay, for you, citing that your diabetes isn't going to go away or wait for a few months whilst they sort themselves out. By doing this, your employer may have heard a bit of an alarm bell, and they may be concerned that they don't know enough about your condition(s) in order to ensure they are not breaching any regulations or requirements under a wider company policy, or disability laws.
Before you pressed for the changes to happen without delay, was there any sign such an occupational health review on the horizon?
At this point, in your shoes, I might be kicking myself for putting my head above the parapet, inviting this interest, but it's done now. I wouldn't think you should be too concerned at this stage. The review could well just go like clockwork, and give your employer a better understanding of your condition. The review could well express your request for an immediate change, or could firm your employer's resolve to delay until replacements are found.
Have you asked your line manager, or HR why the review is happening? I'd start by doing that. Don't be confrontational, but if you tell your line manager you're a bit surprised by this, hopefully he may be able to settle your misgivings.
Regarding the review itself; I would suggest you request copies of all the records making up the review, so that would be copies of any doctor's report from your GP/Consultant, and the paperwork for the interview/examinations you might have. That way, you will have sight of anything they are using to make decisions. In order to ensure such requests are kept to a reasonable level, your employer may use their right to charge you for copies of this paperwork. It's usually only around £10, and I must warn that you will only be give sight of details relating to you. You will not be given access to any paperwork where you and other employees are discussed.
As I say, I wouldn't be panicking just yet, until you understand why it's all happening.
Let us know how things progress.