Best practices for making large changes to P&L (GL codes, subaccounts, etc.)

Hello - I started at a new company and the accounting is all over the place with the P&L being a real mess. I need to create a more logical order for revenues and expenses, as well as create and determine the proper accounts and sub accounts for each of the account numbers.

My question is what is the best practice for this type of situation and what are the second- and third-order effects that I need to look out for especially since there is over 10+ years of past data? Should I just change the account number/name/subaccount of the existing account to the new one? Should I leave existing accounts alone, create new accounts, and determine a date I want to start recording data in the new accounts (like first of the year)? Some other option I have not considered? What should I be on the lookout for? Thank you!