Replacing a harddisk is not that hard. You do have to have a bit of know how on 'what next'. Those you can obtain through some reading , maybe a youtube.
The story that an i7 laptop cpu is the same as an i3 desktop cpu, is partly true, if you stay in the same model number. You can buy a higher model number.
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Usually, replacing a HD is allowed under warrantee, differs per company. That it is allowed by some shows you how easy it should be.
If you're gonna let it be done by your reseller, check the pricing on their harddisks. Compare it to what you can find on the net. Sometimes it saves a lot of money.
Also, I'd advice on an SSD. They're not that expensive anymore.
I usually pick a HD from the leaderbord of
StorageReview.com - Storage Reviews . Pick a HD for what I need it for. SSD for OS and program running, 7200 rpm (less battery) or hybrid for data storage (might even be worth considering a green or blue disk). Could also go with an external disk for data storage.