TEB,
Let me frank by saying not a solution in hand available (at least with me or this response offers any helpful content to you) though this is a very common issue while the new API's being developed. And note sure about Developers how do they deal to test, but for QA/Test engineers, it is really a challenge and time consuming in order to deal with this situation. Once the API's are stable, this won't be requiring, I guess.
Let me re-iterate what I understand from the posts.
You want to update the payloads & headers as per the updated definition in the test steps of your project, which has large number of those. Correct?
And looks you already put your think tank a head and talking about map and all.
But for example, there is a method say "add":
1. When the definition is updated, does it not update all test requests? or at least does not it ask so? Because, it asks so for the case of WSDL / SOAP.
2. If "add" method is used in n number of test cases, does all the steps have same request (may be I am referring to some optional elements)? If all the requests in test cases needs same request, then job is simple. Otherwise, job is more complex.
3. Are you using Swagger Definition? or some other format? please specify.
Looks you are using SoapUI Open Source edition and #1 may be obsolete as it won't generate any payload for REST.
By the way, how are you managing to generate payload for each method in your case? This is very interesting and glad to see if it is a generic solution and share if possible.
I did spend some time generating the pay loads automatically for REST in Open Source edition (like how it does for SOAP requests), but that was limited for that specific definition in JSON format with example values, not a generic solution. Had to use some external libraries and implement some listeners. This is where Pro is good generating the payloads automatically and people pay for it. If we want to implement our self, then need lot of programming skills and time to do so, but achievable.