... to a 500GB SSD (like size as present HDD)?
That's about the limit, as it's already maxed out at 4 GB RAM.

Guys, here are the specs for this machine.

It's steady, runs Win 10 flawlessly (so far), but it's a bit slow on certain disk operations, like when:
- booting: 1:30 to Desktop 2 min to last startup load (which I've pared down a lot already)
- launching Firefox: mostly 10+ seconds to full open
- " Faststone: " " " with only a few C: drive items to display.
- and "other stuff" too...

Here is the WinPC EI score...
ChrisPC_WEI_Scores_2019_08_30_110058.jpg

Note WEI overall is 6.5 on my own laptop, with 8.3 SSD score.