Recently I left a startup I was working for to move to a large software company.
In addition to the good advice others have already given, let me also encourage you to check the terms of your current employment for potential conflicts. Some employment agreements stipulate that any code you produce -- even on your own time -- is intellectual property that belongs to the company. You also want to avoid getting into legal trouble with your current employer in helping out the old one.
-xdg
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