Follow this steps:
- Connect your new drive as an external driver, with a SATA cable to USB (usually 3.0, but you can do that with two usb 2.0 ports also, like this one.)
- Clone your drive to the new drive with a tool like Disk Copy, but there are also other free tools you can try out.
- After cloning is completed, remove the old drive and insert the new one in it's place.
- Turn on your computer (running the windows in the new drive now), and connect the old drive with your sata to usb connector and format it.
- Insert the old drive in another free slot you have on your computer.
If you already cloned the old drive to the new drive, but only make the new partition as the boot drive, and change it's letter, follow this steps:
- Go to start, and in the search box type "cmd", right click>run as administrator.
- Once opened, type "DISKPART"
- On diskpart, type: "list disk", then "select disk 0" (since drive D:\ is the one you want to make changes, as I see from the screenshot)
- After that, type "list partition", then "select partition 1"
- type "active"
And this is where you have done the partition bootable. After this, you have to go to Disk Management and right click to Disk 0, change driver letter to whatever you want. Then, right click, change driver letter of "C:" to "D:". The final step would be to change the driver letter from whatever you assigned to, to "C:". So in other words, you make the drive bootable, change the letters, and then follow the steps above but this time with the old drive, and make it "inactive" (you can change the driver letters from diskpart also. reopen it, select the driver you want to make changes, type "assign letter=?", where ? is the letter you want)
This can result in problem with booting, and sometime cause the computer to launch the startup repair. Therefore, I suggest you the first method.
In case if you are unable to change the driver letters, make the new partition as active, and the old one inactive, restart the computer, and then change the letters.
Using a live cd application, like Clonezilla, which basically runs from a CD or USB, and does the operation as working from another system. Select the source drive and the destination drive and complete the operation. Everything should work fine then (even though clonezilla has some limitations in drive sizes). But that is for cloning. After you remove the old drive from it's slot, and check everything works fine, you can format the old drive.
In case I made it unclear, or if something is wrong, please let me know, I will try to help you. But please, I would recommend you stick with the first method, since it's more secure.