The Terms & Conditions state that the minimum subscription term is 12 months, therefore, I suspect you can't do a switch mid-term, not that you were thinking of doing that anyway. But, my Zinio subscription, for example, appears to be arranged through Zinio and not ImagineFX (Future Publishing) therefore it's a whole new contract, not really a renewal. I don't know if it's the same with the other digital versions, however.
The good news is that there is no real danger of missing a digital issue because they are instantly available once you subscribe unlike the print edition where you have to wait a month or two if you don't time it right. You can safely let your present subscription run down and then move over to digital as soon as the next issue is released.
The bad news is that if there's a problem with the subscription, I have to go to Zinio, not the nice people here. The good news, where Zinio is concerned, is that they appear to be quite responsive.