You're good, it's an honest question to have because it's a recognized glitch with the older automation series builder. The only way to be able to edit a step's header would be to copy a normally-created email into the series.
At this point, since your welcome email was activated and sent out, I'd say it comes down to how important it is to make sure that subject line and preheader are changed for your future sign-ups going forward. If so, I'd advise making an email for your welcome auto, then copying that into a newly made series.
Another option, which I tested in your account just to check its availability, would be to use the newer Automated Path Builder, which can be setup for a welcome series, as well as more complex and longer-running sendouts in accordance with the welcome. You'd could create a new one, or use my tester called "CSCTEST_welcome" and rename/edit as you see fit.
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