For what it is worth, I have done this on two separate occassions, and everything seemed fine. Manually changing the state of the fiscal year to draft in the database directly, then running the wizard to cancel the closing entry seemed to work. . But you _Should_ not. No one reading any of the rest of this post can  or should hold me responsible for anythign that happens if you do - generally a fiscal year should be closed only once you have submitted data to a governmental or tax authority, and Odoo has worked to make this immutable (as it should!) after the FY is closed. 

Update account_fiscalyear set state ='draft' where id = YOURFISCALYEARID ;

Find the view for account.open.closed.fiscalyear (in stock odoo 8, name may change in other versions) . Update the search domain to find the FY if necessary.

make sure you cancel the opening journal entry, which means you need the account_cancel option

Run the wizard , which should remove the opening entry, as well as the reconciliation between the closing entry and the opening entry.

And then, I think, you are good. You can re-open periods in the formerly closed fiscal year. I closed a FY prematurely once, and while I could have restored from a backup taken 5 minutes before, I wanted to see if a process could be made to work. I Absolutely understand why Odoo would not want to encourage or even publish data about this. Taxes are serious stuff.

Close a Period:

(Click the Close Period button)

Re-Open a Period:

(Click the Re-open Period button)