What’s the best practice when you want to change the segmentation of a transaction in a closed period? Reopen the period or book a journal?

Well it sort of depends I guess, how far back it was, what country (some EU countries it is illegal to open past periods) - I think in general booking a journal would be considered best practice because it gets the job done, is 'safe' and so on. In reality, say just just closed, maybe reopening and making the change would not be a disaster if done off-hours (so no one posts anything while this period and subsequent ones are open.) If you open once rather than never you also need to have that discussion every time 🙂

yeah, the best practices are to NEVER open already closed period. Just do the reversal and rebooking in the current period.