NetSuite confirmed that it can only be done manually or through CSV import. Recalculating the previous values is a nightmare.

Brutal. Did they confirm the exact steps to take, by chance? ie: I know there are 'BG' custom records to delete, plus the journal... but are there others?

No. They didn't. I did an initial test. I just updated the net book value, last depreciation period, last depreciation amount and last depreciation date. Seems to be working but too early to conclude.