Yes I know you must first upgrade your current legit Windows install to Win 10 to be able to do a clean install. However I cant find any info on how the clean installs will work. Will M$ require you to have a MSA account so they can email you a new key tied to your hardware? Will there be generic iso's available to download like DigitalRiver with Win7? Will it be like "Media Creation Tool" where M$ will create a modified ISO with your key embedded? Is there a possibility you won't be able to do a clean install on July 29 since M$ said they aren't going to release Win10 all at once and some will have to wait a little longer to get their copy?
Retail always allows a clean install but can also upgrade too as long as the previous version supports it ....
and just to make sure it penetrates thicker minds. you must upgrade before you can do a fresh install if you intend to activate with a free upgrade. if you are using a retail key (when that day comes) you will not need to bother with the upgrade process.