Basically, I'm wondering if I have to interview again, or if they'll just take me back. Has anyone gone through the process of going back to a previous big corporate employer?
Generally speaking, if both the company and your previous manager regret your departure and want you back, you can go back pretty easily.
I know, for example, that Google generally* does not require an interview for a return to the same job function and job level within 12 months, provided that the departure was "regrettable" and the outgoing manager and at least one or two internal references approve of the return.
After all, you're more valuable now than you were then -- you appreciate what you had now in a way that you didn't used to. So, if they already were sad that you were leaving, they should be even happier to have you back just a little bit wiser.
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* Generally. This only applies if there is currently-open headcount for you where there is a good match.
I had a colleague leave for a start-up, hate it within one week and was reinstated into her old role 10 days after "quitting".