Hardly an official answer, but here's my take on it.
The upside of these edits is that they make posts look better. The downside of these edits is, as you noted, that it will bump old posts. So the question boils down to "is the tradeoff worth it?"
Personally, I'd say no. yes, those posts would look better with the fancy new markdown tables, but if they're buried in history, who even looks at them? My two cents - if you come across an old question that got bumped somehow, and have the time and energy to edit its tables formatting, by all means, go ahead. If you intend to actively search for questions with malformed tables and get them bumped, you risk drowning out the front pages with questions that really have no new information added to them - only the improved formatting. I'd refrain from doing that.