There is no such thing as "adding requirements" to a fixed-price milestone.
There is no such thing as "changing requirements" for a fixed-price milestone.
In my opinion:
As soon as a client tries to do something like that, the free should immediately stop working on the project. The client needs to either:
a) release all escrow money and set an hourly contract
b) the freelancer files a dispute for non-release of escrow payment and the client loses access to the freelancer's services
Fixed-price contract do not mean indentured servitude for the freelancer. Freelancers should be very strict about how they use them, and ONLY do the work written in the task agreement.