when you upgrade a revit model to a newer version, the first thing you want to do is open and detach from central and save an archived version of the original model.
If possible try to have all users out of those models during the upgrade process
if you have links in your model you will need to upgrade them as well.
if you are using a Revit Server you will need to be aware of a particular folder structure by version. (it may vary from your Network)
always upgrade sequentially do not skip a version, it will cause issues later on. 2016, 2017,2018 etc.
I open and detach from central preserve worksets and save as to an archived location. during the save as you want to confirm you are saving this as a new Central file in the correct location.
see options in the save as window

after each upgrade be sure to save as and confirm the location is actually saving a New Central Model
open the original in the next version to upgrade to that version.
repeat steps to archive the new version.
repeat steps to upgrade to the next version
repeat these steps for all your linked models as well.
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