This is one of those types of situations where there's really no right or wrong process for the user to follow. The "comfort level" of the user is really what's controlling in terms of what approach to follow.
As far as I'm concerned (given your situation), I would upgrade to Win 10 from my present Win 8.1 OS installed on my present HDD boot drive. The important (actually, critical) consideration is that the current OS is malware & bloatware free and you're perfectly satisfied that the OS, et al, functions problem-free.
Following the upgrade and determining that the system is properly functioning with the new Win 10 OS, clone the contents of the HDD to the SSD using the "data-migration" (disk-cloning) program bundled with the SSD should one be furnished by the SSD manufacturer. Otherwise you could utilize one of the freely available disk-cloning programs.
Disconnect the HDD from the system and boot ONLY with the SSD connected (preferably to the motherboard's first SATA port (connector), designated either SATA 0 or SATA 1. After working with the SSD for a reasonable period of time to determine all is well, reconnect the HDD and manipulate whatever data you desire on the HDD secondary drive.
On the other hand, should you be more comfortable following the suggestions re fresh installing the Win 10 OS onto the SSD, etc., so be it.
(Actually our process always first involves cloning the contents of the HDD (the current system drive) to another disk as a complete backup of the system. Then should anything go awry during the process I would follow (as indicated above) I would always have a complete backup of my original system. We ALWAYS follow this course of action anytime we contemplate making significant/substantial changes in a current OS).