There's multiple ways of going at this:
http://www.howtogeek.com/224342/how-to-clean-install-windows-10/
This seems like a reasonable source that may come of use, if you have your previous Windows key, then apparently (according to that source) it should activate your Windows 10, whether or not this is true I am not sure, but it seems reasonable for the upgrade.
You can also clone your HDD (The partition for the OS) and clone it onto the SSD, for this, you will most likely need a repair disc to go along with it.
For your final question - You can remove Windows 10, as it will/should detect the OS on there, and it should remove the major programs on there - Just clean up your HDD with the built in feature and it should remove the OS files.
PS - You may want to do a full backup before you go ahead with this.