There is a thread in this forum to show you how to download the ISO file. The Retail version and the OEM are the same, the differences are the product keys. I am not sure if you can download the ISO using the OEM product key.
Once you have the ISO then it becomes easy, just swap the drive currently in your laptop with the SSD and install it. The other thread that I refer to earlier contains info on how to save your Activation Info and also retrieve the product key.
The version currently in the Recovery partition from ASUS is also legal except they bundled with a lot of crap wares.