According Stefania Duico, the lead of Windows Phone Italy, the Windows Phone Tango update will be officially called 7.5 Refresh.
As you probably know, currently WP Mango is called just 7.5 and now the WP Tango was 7.5 Refresh.
Adding the Refresh moniker and not changing the OS version actually makes sense. Though WP Tango or WP 7.5 Refresh is just another configuration of the WP Mango which is called the WP 7.5 the feature is that they lowered the Windows Phone specs and this is their major role and that so less-capable devices could run it (without multi-tasking, live tiles, etc.).