You would be bonkers not to tell your insurers, or you risk a potentially denied claim should anything unexpected occur whilst on holiday. Your insurance contract is sealed as soon as you have paid your premium and the cover has begun, so they should be unable to vary their terms for this year. I say should, because without reading your specific terms I couldn't be more definitive, but I would say it would be extremely unusual if they could.
As for next year; I would say that provided you are being cared for and monitored you shouldn't have any more problems getting cover than any other diagnosed diabetic. Whilst you state you're not under treatment, I would assess that you are. You have been diagnosed, and current treatment is careful home management and monitoring to assess your medication requirements. That that assessment currently states you need no medication is just as valid as giving you medication.
When the time comes to renew, there have been several threads on the forum by people with similar concerns about their recent diagnosis, so there would probably be some value conducting a couple of forum searches to pick up some of the little gems that appear from time to time.
Good luck with it all, and enjoy your winter holiday. I find it a wonderful to to escape the cold and top up the Vitamin D, although you may be going to freeze your bits off atop some mountain somewhere!