It doesn't really work that way. I still remember the previous timeline even when I alter the past.

I was trying to cause as few alterations to the timeline as possible, which is why I knocked out past Barry rather than talk to him. The reason I had to go back was because the person I needed to talk to was dead by my time, and telling past me that would've altered the timeline more than just punching him out did.

Of course, he woke up before everything was over, so the future was altered anyways, but it's the thought that counts.