Changing Fiscal Year for 75 companies

My company was purchased this year and will be changing their fiscal calendar in July of next year. We only run Financials (4, 4, 5 periods) and procurement. I have seen some postings about changing G/L fiscal year and others about changing Fixed assets fiscal year. I need a process that works for both. The current company is moving from April - March to July to June. First concern is the three months extra. They had requested a 15 month year, but that will not work. We have data back to 2000 and