You can move a slider as soon as it appears.
Indeed, if you move it during run without hitting Stop. However, if you click Stop it's a different story. See below
For testing a script with a certain slider position, click [Test], wait until the sliders appear, then click [Stop], put the sliders to the desired position and click [Test] again.
I've tried this with my script and logged every step. It doesn't work if I click Stop immediately after the slider appear. I change the slider but after I reTest a script the slider returns back. Above mentioned condition isn't enough. One should wait a bit longer and then hit Stop.