What grammatical structure is the italicized portion of the sentence?
although her personality had not changed at all, megan looked quite different.
adverb clause
infinitive phrase
past participial phrase
elliptical clause
adjective clause

If the italicized portion of the sentence is Although her personality had not changed at all, then the answer is adverb clause. 
It is not an infinitive clause, because there are no infinitives. The same goes for participial phrase.
It is not elliptical, because this sentence has a verb.
It is not an adjective clause, because it shows the contrast between her unchanged personality and her different looks, which is an adverbial characteristic, not adjectival. 

Dbecause it makes sense