what happens as the language that the script represents changes? redesign?

I suppose that as words change in pronunciation, their "spelling" will change. (Something which did not happen in English after Chaucer's time basically.) If the phonology of the language changes, i suppose new characters could be designed. Not sure how the folks at the Unicode consortium will feel about that.

And, by the way, how do you pronounce ताज महल? I've always said /taʤ məhəl/. Do you say it with a /z/ instead of a /ʤ/? (And, now that I think about it, my त is more alveolar than purely dental.)

Ceci n'est pas un seing.