In most cases, you don't need a new license.
If you have a retail key: You won't need a new key, just type in the key when you're installing Windows 10.
If you have an OEM key: You will need a new license IF you are switching to a new motherboard.
If you upgraded your old Windows version to Windows 10: You won't need a new license, install windows 10 without a key and it will activate automatically.
You can switch your Windows install between disks, using a migration tool. However this is more than a hassle and will break things if you're using a lot of new components so a fresh install would be better.
Legitimate Windows keys can also be bought for a reduced price at certain 3rd party websites.