The first thing to do is to read your policy document and it will tell you if you have to notify them of any changes. If you do then it is best to tell them.
If you don't tell them and anything happens they may not pay your claim, and as it is Cuba I expect it would be very expensive if you had to pay for any treatment yourself, or even worse get repatriated to the UK.
I don't think it is likely to cost too much, but if your current insurer wants to charge a lot it would be worth ringing one of the specialist companies.
Can anybody advise me on travel insurance. I'm iff to Cuba in march for my daughters wedding I've got my normal insurance. Now I've had Dsci should I tell the travel company as this was only pre cancerous cells and how much more would they charge. Really don't no what to do as by then I would of had my last appointment with the doctor. Pink lady.