Question Advice on installing an ssd boot drive in an existing system

May 7, 2020
Not 100% sure if this is the right place to post this, but idk where else would be better.

I have had my current laptop (a ge72 2qd apache pro from msi) for a few years now, and the hdd it came with has slowed down to a crawl, spending most of the time with disk usage maxed out at 100%. After doing a bit of research, I decided to try installing windows on to an ssd. I got myself a relatively cheap 256 gig m.2 ssd (HP ex920) and was getting ready to install it, but almost every guide I have seen lists reformatting your existing hard drive as a step, and either expects you to have another external drive you can back everything up onto, or that you are fine losing some of your data. Its very possible I've misinterpreted them somehow, as I am only moderately tech literate, but given that I have ~800 gigs of data, built up over multiple years of use, I'd really like to avoid losing that if possible.

So I guess my questions are this:

Is it possible to install an ssd into a system with an existing hard drive without backing up/losing data?
Why is it recommended that you reformat the hdd in the first place? Is it something that I should just suck up and do?
Having read through several different guides, all with slightly different information, is there any specific source you would recommend for a guide on installing an ssd into an existing system?