Changing Ssd At Same Time Upgrading Win7 ->win10 - Questions

I have a DIY Win7-64 computer with a Samsung 224Gb SSD and a WD 250 Gb HD. I also have a 1T external Seagate USB HD. I'm going to bite the bullet and upgrade from W7 to W10. At the same time I'd like to upgrade the SSD to a 1T SSD. My data is spread across both the SSD and the internal HD, and my plan/hope is to put everything on the new SSD and use the internal and external HDs for back/deep storage/whatever. I am planning to use the Microsoft digital upgrade path, if this is workable [I just used it for a ThinkPad notebook, and the upgrade went well]. QUESTION: how do I sequence the SSD replacement and the OS upgrade? I can imagine all sorts of back-and-forth maneuvers between the drives, disk image backups, OS changes, etc. What's the school solution, here? Thanks in advance.
...Niky