Periodic Inventory and COGS

I have been posting all purchases to COGS accounts for the past five years. I take periodic inventory (usually just at end of year, though would like to start doing it quarterly.) I've been handling this separately in Excel for end of year COGS and tax purposes, but would like to be handling it correctly within Quickbooks Desktop so that I can run accurate P&L statements. 

What is the correct way to handle purchases, COGS and inventory? I've seen this recommendation: 

https://quickbooks.intuit.com/learn-support/en-us/reports-and-accounting/journal-entry-for-periodic-inventory-and-cogs/00/248631

So if I understand correctly, purchases should not be posted to COGS, but rather an asset account (Purchases). When I take inventory, I do a journal entry to credit the Purchases account (adjusting it to the inventory amount) and debit COGS account (for the inventory amount). 

Next question: is there any way to adjust the past five years of data in Quickbooks Desktop so that I can enter the last few years of EOY inventory and then run P&L comparisons? I have various sub-accounts of COGS - materials, packaging, production costs, etc. Is it as simple as editing those accounts and changing them to be subaccounts of Purchases? Or does that not change the existing entries? 

Thanks.