This comment isn't intended to help, just reminiscing what I had to go through when I upgraded.
When the Hard Disk in my laptop start acting up I had to finagle the thing to get a smaller partition so I could move it over to the SSD. I nearly pulled my hair out getting it to work. Maybe you remember that, maybe not...
Pi suggested 1 or 2 programs that ended up helping.
But moving things from a larger Drive to one that is 1/2 capacity isn't your issue, so, eh.... Good luck.