When you try to change the period status to Closed - the system inform you that you are trying to close it and no entries can be passed in the closed period. which informs and avoids doing it by mistake.
Don’t try to do the same mistake - because there are some validations build in the system.(you cannot change the period status when it is closed). You can change it to stopped if you want to restrict some one post in that period.
And finally i will suggest not to do the changes directly in SQL database because the validation and business logic will not be applied or done by the system.
I have heard of back end change to open previously closed period. Whichever option you choose, make sure it is tried/tested in testing environment.
Also afterwards remember to run ‘recalculate period balances’ ( under GL → Periodic) before you do any reporting. This job will delete underlying transaction tables and then rebuild the data. Depending on the volume of data, for obvious reasons this might take a while.