Could anybody let me know whether their is a difference is use between the two? Sofar from examples i have seen they seem interchangeable.

Thanks,

Marleen

I think there is a stronger element of transforming or reshaping one object into another with 变成, and it's used this way in 口语 often. For example 'tadpoles will turn into frogs', 'the class changed into an English class' (context: a chinese class, where the roles reverse by the end of it!).

蝌蚪会变成青蛙
这节课变成英语课了

变为 isn't really said as much. Where it's written, it's more like 变 + 为 + something. With 为 as a verb ending. I don't think there is much there in the way of meaning.

Eg. (转变 + 为) 蒸汽机使热转变为能

Of course, this is only based on my understanding so far. I may be wrong!