Moving from Platform Update 15 to 22

Hi all,

I have some questions about moving to latest Platform Update 22 from 15. Can someone please help me out and answer the following questions:

• We are currently on Platform Update 15 and want to move to the latest Platform Update 22. Can we still do it simply by applying Platform Update 22 package (from LCS) over the environment?
• We need to get rid of all the code over-layering (if any) before updating to PU-22? We can do it either manually or running code upgrade service on our codebase on Azure DevOps.
• As per One Version FAQ (https://docs.microsoft.com/en-us/dynamics365/unified-operations/fin-and-ops/get-started/one-version) all customers on 7.x application release must need to upgrade to 8.1 before April 2019.
  So even if we save ourselves for now by simply updating our platform version we still have to perform the upgrade. 

Thanks.
-Moin