Hi Solicitors,
Yes, you can swap the 512GB SSD with the internal hard drive.You can buy any 3.5-inch SATA SSD that will fit in your system. You may refer to the Owner's Manual for instruction on how to replace the parts on this system. Please follow the safety instructions before replacing any hardware components.
Also, you may go ahead and upgrade the mSATA SSD drive from 32 Gb to 256GB, but Dell has only tested up to 32GB mSATA with this system. The purpose of having mSATA SSD is to accelerate a conventional hard drive for a faster boot. However, you cannot format them together as one drive, because they are 2 different drives.
Hope this helps. Please reply if you have any questions.