We upgraded straight from M5 to M7 (Windows servers but don't see why it would be different for any other OS). We did it with help from a specialist from SAS though.
I assume you want to do an upgrade in place so you will need to bear in mind that SAS will be out of action for the duration of the upgrade. Typically this is a multi-day job so your management need to be OK with the downtime.
The SAS Users Group for Administrators (SUGA) is open to all SAS administrators and architects who install, update, manage or maintain a SAS deployment.