This is similar question to your one here. I am My first reaction is that the antecedent of 'which' is 'change', which would make the verb 'serves'. However, there is no guarantee of this.
It's the sort of thing that near-senile old teachers write to The Times about, deploring the lowering of standards. Most of us just don't worry; indeed, many of us would not even notice which form of the verb you used, particularly in speech. It's rarely significant; i.e., whichever you choose, the overall meaning doesn't normally change much.
This is similar question to your one here. I am My first reaction is that the antecedent of 'which' is 'change', which would make the verb 'serves'. However, there is no guarantee of this.
That was my first thought as well. I read it as not being the circumstances which serve as a cause for investigation, rather the change in circumstances which serves as the cause.