Your original SSD contains a functional version of Windows 7 and if Windows 7 came pre-installed with the computer it also has a “Recovery partition” which can be used to restore Windows. I recommend that at the very least you leave the drive disconnected and untouched for a period of time until you are satisfied there are no issues with the new SDD.
Your other option is if for any reason you need to return the PC back to the manufacture for repairs, remove the new SSD, connect the smaller SSD or hard drive which has the factory installed version of Windows and then return the PC.
Finally, if you have a desktop computer the smaller SSD can be formatted once you are satisfied the new SSD is functioning properly and used it for data storage. Some laptops also have a second drive slot so again the smaller drive can be used for data storage.