[RPG] If A Changeling becomes a Succubus, can she fly

No, you’re not really missing anything

With regular specialization, as long as there is at least one spell from your chosen school you want to cast each day, you can do that and use your regular slots for any other schools you might be interested in.

Dual specialization only helps you if you cannot do that, and have some spell level where you really don’t want to cast any spells from the school you’d choose first.

Which might happen, maybe, sometimes, but not often; certainly not often enough to justify banning a third school.

Changeling wizard? Recaster!

Seriously, recaster from Races of Eberron is awesome and you should seriously consider taking it. One of the very few cases where losing a spellcasting level is justified. It’s flavorful and powerful.

There isn't an officially stated lifespan for Changelings (as of writing this answer).

The most important thing to remember is that Unearthed Arcana material is only effectively playtest material. It's not "officially" published or polished and thus isn't nearly fleshed out as much as the core rule books or supplements so you're not likely to find that level of detail until it's actually put into a supplement book, if it even does.

There's only one other mention of changelings that I can find and that relates to doppelganger children in the Monster Manual but even that doesn't give us any information on their lifespan.

However, if you really want something to base it off and get a rough idea then it has been detailed in the 4e Eberron Player's Guide (unfortunately I can't speak for older editions):

Changelings reach maturity at about fifteen years of age, and they live about as long as humans do.