Sharepoint - Change default form in SharePoint Designer
You can set it as default from SharePoint Designer 2013 as well. You need to select the form and then it will enable the "Set as default" button control in the Ribbon.
Using powershell, should be something like:
$web = Get-SPWeb http://path/to/web
$list = $web.GetList("/server/relative/path/Lists/ListUrl")
$list.DefaultDisplayFormUrl = "/server/relative/path/Lists/ListUrl/displayifs.aspx"
$list.DefaultEditFormUrl = "/server/relative/path/Lists/ListUrl/editifs.aspx"
$list.DefaultNewFormUrl = "/server/relative/path/Lists/ListUrl/newifs.aspx"
You cannot change these properties from the UI.