I am assuming they are being sneaky and wanted to wait until all the jan 1st renewals have been locked in together with the expiry of their 14day cooling off period.
My feeling is that VHI are a reputable company and would not attempt to reduce the level of cover afforded under the different plans (increased excesses, shortfalls, procedures covered, hospitals covered etc) under cover of the changes. Best review your current coverage on your existing plan and compare to the reduced price offerings in March. Note that the HIA website does not hold onto old versions of plans for comparison.