What if I reverse a Project Cost Allocation in a closed period

Summary:

What if I create an Project Cost Allocation rule, run the Allocation for 2 months, now we are in month 3 and i decide the rule wasn't right and i want to reverse the allocations generated in last 2 months, fix the rule, and rerun for the last 2 months.  At this stage the last 2 months are now closed periods.

Would I have to reopen the periods for that to work? Or will the reversal be generated for the first 2 periods and posted to the current/third period? Then can I change the rule and rerun it for the first 2 periods .. but again, will it post to current period, or would older periods need to be open? How will this play out?