You can change insurer at any time, not just renewal. However, changing plan mid-year depends on the insurer. I think VHI have something in their rules about only being able to upgrade at renewal, but I don't know if they apply this in practice.
A few issues to consider:
1) All health insurers specify that their cover is only for people living in Ireland for a total of 6 months or more in the year. They may be entitled to refuse claims if they suspect that you are abroad for more than that and may ask for proof of residence. When I was working in BUPA, this was mainly to prevent people making claims for treatment abroad, which is a bit different from what you're considering.
2) If you go for the cheapest plan, an "upgrade" waiting period may apply should you choose to increase your cover when you return. This means that if you have a condition before you increase your cover and get treatment for this condition up to two years after increasing your cover, then your claim will be paid as if you were on the lower level of cover. This might mean you have a large shortfall to pay depending on the plan you're on and the hospital you used. So, if you went for the cheapest level of cover, then this is the cover you would receive if you came back to Ireland for treatment. Of course, this level of cover might be what you want, but better to know now than then.
Other options to consider would be getting Global health insurance. VHI offer this kind of cover as do BUPA International. Prices vary depending on age and location.
Hope this helps.