Your wife's no claims bonus will be valid for three years, so it should not be an issue to register you as the main driver. Speaking to your insurer is your best bet though because it may be on a specialist policy and, therefore, might not make the slightest bit of difference. As the vehicle is not the main vehicle being used it should not really change things. However, if your wife is very unwell and might not drive again then it may be best to inform your insurer of these changes.