The TS to CS conversion is possible, sometimes. Since you're going during a Free Dining Plan promotional period, it's likely, however it's usually only possible at your resort. There is no consistent policy on it though, so the best bet would be to ask when you're there (I know, that melts the planning brain a bit).
As far as Walk-ups, they are ALWAYS possible (in general, not for a specific restaurant), however the best bet would be to take a look and see what ADRs you can make now. If you have any Must Dos, be sure to work the online system several times a day to get it. If you're just going with the flow, you can always as Guest Services in the morning to see what's available. Note that inside the park will be much tougher than at resorts! (Except Epcot, since there are so many restaurants in the park, you won't likely get Akershus, Garden Grill, Le Cellier, or Teppan Edo, but there should be a couple options open).
And yes, off times is the best time to do it, but don't be surprised if they're all booked up even at those times!
