- iTunes will whinge when syncing and you'll see an circle with an exclamation point for podcasts with missing audio files.
- Delete these episodes from the podcast then colapse
- Colapse the podcast by clicking on the down facing triange.
- Hold the shift down and expand the podcast. iTunes will fill in the entries that you deleted and the get button will re appear.
- By now you'll have probably realised that you just click Get again
Why they couldn't just add "re download" to the context menu is beyond me, still that's Apple faggotry for you. I don't know if this works on a Mac as they suck and I obviously I don't have one.