If you're talking about me.... that's not what i meant. Your ECU won't undo the tune. Every time you re-tune your ECU goes into "learn" mode (just like disconnecting your battery) and takes a few hundred miles to optimize. The difference is you've set it to target different parameters with the handheld. So it'll still "target" those new parameters, but takes some time to get there. IE with a stock tune, when you disconnect the battery the ECU will take some time to refine itself, but it targets the "stock" tune parameters.