When you purchase an upgrade license we require the original license you upgraded from.
Once the purchase is complete with Fusion 8 your license key for Fusion 6 is upgraded and goes away from the licensing area of MyVMware.
I would recommend purchasing your upgrade to 8 when you're actually ready to deploy it.
You can always check our the 15 day trial if you aren't sure if it will work for you.
Yes you can.
You might even have found the perfect time to do so
Wil| Author of Vimalin. The virtual machine Backup app for VMware Desktop Products
| Vimalin : Automated backups for VMware Fusion and VMware Workstation Professional
| More info at https://www.vimalin.com
| Twitter @wilva
| VMware Wiki at http://www.vi-toolkit.com