ISO you can download from MS site, safest place and they'll gladly give it to you. You don't have to deactivate anything since it's going to same MB. If That W10 you run now is activated all you have to do is to start installation and skip entering reg numbers, it will activate automatically as soon as you enter your MS (Hotmail) account.
Just make sure you download sameW10 version (Home, Pro, Language, 64bit, 32 bit.) as you use now.
When you connect your old HDD and if you don't have anything you want to keep on it, it would be best to go to disk management (run > diskmgmt.msc) and delete all the partitions on it (because there will be several not needed any more) and make a new partition formatted to NTFS. That way you have totally clean disk you can store anything you want on.
You can also use something like this:
http://www.majorgeeks.com/files/details/active_partition_manager.html to have better look at disk and do that partitioning easier.