Will be a problem if I clean install the 8.1 before the intended date using the key 8 pro that is for upgrading from legacy operating?
No it wouldn't be a problem. I used my Windows 8 Pro Upgrade key to do numerous clean installs to both 8 and 8.1
Is that a Windows 8 Pro Upgrade key as in, upgrading existing Windows 8 installation, or an Upgrade key used to upgrade XP/Vista/W7?
I had Windows 7 shipped with my laptop. I took the offer of Microsoft to upgrade to Windows 8 for 15 USD and since I had Windows 7 Pro, I got the Win 8 Pro upgrade key. But these days upgrade keys work on a fresh installation not like in the past where you had to insert your previous OS DVD or key first. It works on a fresh install. The only diff. of the ugprade key is rather than buying it at full price, you provided MS with your Win 7 key at check out so they gave you the discounted upgrade key but it works on a fresh installation just fine