Upgrade to Version 4 before Upgrading to UI Version 15

​It is advisable to upgrade any sites that are running version 3\2007 mode in your SharePoint 2010 to version 4 (SP2010) before you attach and upgrade in SharePoint 2013

SharePoint 2010 sites that are still at the UI version 3 (aka 2007) will automatically get upgraded to the version 4 UI during the upgrade to 2013.  They can then be upgraded to version 15, and this is recommended,

Be careful when checking if sites are at which version via Powershell.

make sure to run this command:

Get-SPSite | ForEach-Object{$_.GetVisualReport()}

NOT this one:

This one WILL UPGRADE to Version 4 (2010):

Get-SPSite | ForEach-Object{$_.VisualUpgradeWebs()}

Technet on cleaning up 2010 environment for prep for sp 2013:  http://technet.microsoft.com/en-us/library/ff382641(v=office.15).aspx

If you accidentally roll up, and need to revert, use this shell

Get-SPSite http://machinename/sites/V3UI | Get-SPWeb "webname" | Foreach{$_.UIVersionConfigurationEnabled=1;$_.UIVersion=3;$_.Update();}

Make sure to note the Name of your web, it might be blank to get at it use the object model and look at the site object model using get-spweb

Technet on Managing visual upgrade – http://technet.microsoft.com/en-us/library/ff607998(v=office.15).aspx