One of the in-jokes on the Maiden DVD's is to have a guitarist play another guitar every time they show him. For example, take a look at Rock in Rio or Flight 666 or whatever - Janick's playing his black Strat, then he's playing his white Strat, and then black again - all in the same song. So, you have to be aware of that the footage isn't actually taken there and then; they edit in lots of thing - Rock in Rio was edited with shots of another show etc. As for the newer DVD's they've filmed several consecutive shows so the final result you hear/see is an edited together concert consisting of the best parts, I would assume.
That's the visuals, as for audio - they naturally edit and mix the material in post production. How much? Hard to say, unless you have another copy of the same supposed show to compare it to.
If they'd need to overdub something in the studio today, I would guess that show isn't suitable for release. Was different back in the 70's or 80's when it probably cost a lot more money to record a show on tape. I'm very suspicious of the older stuff, in that regard.