This is another example of common Japanese terms being given a specialized meanng.
HEN-KA (変化) basically mean a change in state or circumstances, whereas OU-YOU (応用) means an application of something to a different situation. For example, in linguistics HEN-KA refers to declension or conjugation of nouns and verbs, whereas OUYOU GENGO-GAKU is usually translated as Applied Linguistics, i.e., the application of linguistics to a practical situation like teaching. 科学"I新'm識の産業--竭閧ﾖの応用: 'kagakuteki shinchishiki no sangyou mondai e no ouyou' means the application of new scientific knowledge to industrial problems.
If you apply this to aikido, OUYOU-WAZA is the application of the same technique to a different situation, whereas HENKA-WAZA is changing from one technique to another completely different technique. However, I have found that different teachers apply this distinction differently.