When Monero implements RingCT, how will the transition be handled?
In particular, how will pre RingCT outputs be migrated into outputs available for use in RingCT transactions?
When someone uses a rct transaction, the outputs are necessarily rct. From hard fork 4 (expected in or around march 2017), both rct and non rct transactions will be permitted. From hard fork 5 (expected in or around september 2017), non rct transasctions will be permitted only if they are spending unmixable outputs (ie, the rules for allowing mixin 0 now).
If you want to make all your outputs rct, you should be able to use
sweep_unmixable first, then
sweep_all. Assuming you do not have a large amount of tiny inputs, that should make your whole balance only in rct outputs.