Hello,
I do not think that what you need can be done on spanning volume without backing up and reformat. The data is wrtitten sequentially - when first disk fills up, system moves to second one and so on. That means you have no control over what is written where, and as there is no redundancy, you cannot replace disks one by one and then rebuild volume after each replacement as it would be with raid 5 for example.
You may want to take look at below thread - not completely related as it deals with disks expansion on RAID 5, but it might give you some ideas.
