How to override User Defined Variable from JSR223 PreProcessor in Jmetr?

I have a such problem

Here is my User defined variables: https://i.sstatic.net/DTlU4.png

And here is my script from JSR223 PreProcessor:

def params = []
def attributes = [:]
Random random = new Random()

def int variantToStartFrom = Integer.parseInt(vars.get("START_FROM"))
def int increment = variantToStartFrom + Integer.parseInt(vars.get("INCREMENT"))

for (int i = 3; i <= 3; i++) {
  for (int j = variantToStartFrom; j <= increment; j++) {
    def attr = [:]
    attr.put('location_id', i)
    attr.put('variant_id', j)
    attr.put('quantity', random.nextInt(100000))
    params.add(attr)
  }
}

attributes.put('items', params)

def json = new groovy.json.JsonBuilder(attributes)
vars.put("START_FROM", increment.toString())

sampler.addNonEncodedArgument("", json.toPrettyString(), "")
sampler.setPostBodyRaw(true)

As result I'm trying to run Thread group two times

My idea is to override my START_FROM var in User Defined Variable so then new Thread group run will use START_FROM + 1 value

Is vars.put("START_FROM", increment.toString()) should work as I expected? Or is there any other way how to override User Defined Variable from script?

It works as expected. Your script will change the value of START_FROM after running script, but it only happens while running. When test finish and re-run the test, STAT_FROM will be back to value that defined in GUI, it's is 1.

To sum up, you can change the value while running test, you can't change the value you've already typed in GUI.