In OneWorld, the major shareholder was "inadvertently" setup as parent company (top level) complete with its own set of transactions, what's the best/least painful way to fix it? Note that its subsidiaries have transactions already. And the second level sub should be the parent company.

There's a hierarchy change tool in General Preferences that you can use to flip them around. If there aren't any eliminations on either sub and all your subs are the same currency, it should work pretty easily. If both of those things aren't true, it's going to be trickier. The FX stuff may mess up your CTA account and elims may need to be re-run after you do the hierarchy change and may come out differently. Regardless of your scenario, I'd test extensively in Sandbox first.

Great inputs! Would love to test but we don't have Sandbox for now. Thanks again!

i doubt you can wait till you have a release preview... but if you have not upgraded yet - consider that approach

I'd consider buying one for this change. NetSuite Support might not be very helpful if you try to do this in Prod without testing in Sandbox. I understand Sandboxes aren't cheap, but the alternative is trying to do a restore if something gets FUBAR'ed and I think those start at 10-15k.

I checked the field that allows changes to the sub hierarchy and it seems that there's a date restriction (max of 30 days) which isn't very clear what date is used as reference/baseline.

I believe that just is the date through which the tool is active. Meaning after that date, you won't be able to make changes anymore. I don't think that affects anything about changing transactions, etc.