I reserved my Windows 10 copy, then got a new hard drive and reinstalled win7 (same code). Can I still upgrade to Windows10?
Yes. as long as it's activated and a legal copy yes.
You should be able to, you can get the upgrade tool here
Remember Upgrade to Windows 10 and then you can do a clean install. Source Below,
IMPORTANT: To upgrade for free, select Upgrade this PC now in the tool. You should NOT be asked for a product key during the upgrade process. If you create a USB and use it to install, make sure to remove the USB as soon as setup begins. If you don't, you may be prompted for a product key. If this happens, remove the USB and restart your PC to continue with setup.