I'm sorry but that seems ridiculous. There is also nothing in the rules to imply that they take TWO standards, one normal and one magical. It seems ways more clear that a unit standard bearer who is taking a magical standard isn't trying to juggle two standards, but is using a standard that is magical instead of the normal unit standard. Why would they bother taking two? Or tack on a bit of magic to the regular one? Since there is nothing in the rules to say that they take a magical standard in addition to their regular one, I think it's silly to assume that they stick their normal standard in the back pocket and use the magical one until it's destroyed and then bring out the regular one.
Really doesn't seem to matter. If you buy the standard bearer upgrade you get whatever bonuses that entails. If he loses his banner, which I don't think he does if the magic is destroyed, he just merrily goes on his way and retains the rules that he was bought with.
Errr, but there is P.91 of the Lizardmen army book, temple guard entry states "May upgrade one temple guard to a standard bearer - May take a magic standard worth up to 50pts" Note it does not say "may upgrade his standard to a magic standard", nor does it say "may replace his standard with a magic standard", but "may take a magic standard". Therefore if you choose to take a magic standard you effectively have a standard AND a magic standard, both with separate effects, and destroying one will not have any effect on the other. As far as I'm concerned you can rationalise with fluff and models that however you like, but there is absolutely zero reason why casting arcane unforging on a magic banner would also destroy the army battle standard it is attached to.
The Banner of the World Dragon does not give +1 combat resolution, nor does it give you the Hold Your Ground Rule. Both of those abilities are properties you have if you buy a magic banner or not. The magic item gives the ward save vs magical attacks. The mundane standard gives you the combat res. You're destroying the magic item, not the mundane. -Matt