You have a SharePoint Server 2010 server farm. The farm contains a custom solution package (.wsp). You upgrade the farm to SharePoint Server 2013. You need to ensure that the solution package (.wsp)…

Add-SPSolution adds a new solution to the farm store and will fail if the solution GUID already exists, which is the case after carrying over a .wsp from SharePoint 2010.

Update-SPSolution replaces an existing solution package already present in the farm solution store with a newer version, which is the required first step after upgrading the farm. It retains the solution's deployment state and associations while swapping the package content, making it the correct starting point before re-deploying.

Enable-SPFeature activates a specific feature that is already installed but does not update or replace the underlying solution package.

Install-SPSolution deploys an already-added solution to specified web applications but cannot replace or update the solution package itself in the farm store.