You will only be able to upgrade to Windows 10 Pro 32bit. If you want to install Windows 10 Pro 64bit it will have to be a fresh install.
If it is anything like the 7->8->8.1 upgrade you will likely be asked to uninstall some programs that are no longer supported. Should be fairly few.
If you are running a lot of legacy 16bit applications, I suggest sticking with Windows 7 until those programs can be replaced. Or learn to love Virtual Machines.