Hi Eric,
Legislation regarding posting periods, year end, financial reporting etc. varies from country to country & the functinality in SAP Business One is universal. Changing & modifying while in the active financial year can hence be difficult, but is not impossible.
If, for exmple, you have started your fiscal year on 01.10.2013 & created 12 subperiods, then the end would be 30.09.2014. Now you decide to start a fresh fiscal year to coincide with the calendar year on 01.01.2014. As long as you have no postings in any of the already defined subperiods starting at 01.01.2014, the application will allow you to remove these periods (go into sub-period, right-click & remove) & then begin new period definition.
As with anything that impacts financial reporting, the company accountant must be aware of the changes to be made & make sure that they are compliant with the laws of the country.
All the best,
Kerstin