You will be able to have the hard drive changed out but it is a bit of nightmare so a trained technician should do it if you haven't got the skills, take a look at the iFixit website for all upgrade guides for macs.
Unless your daughter is using pro apps with heavy images or video files or she is using multiple VM's then 8GB should be more than enough RAM for the forseeable future. The only issue with that machine going forward will be the rather weak integrated graphics card, still it performs as a macbook air so the likes of web gaming and older less intense titles should be fine.