The 600 wasn't a temporary fill-in for the 1200 so much as a replacement for the very aged 500. It shares some architectural features with the 1200, along with case design.
Biggest difference between an Amiga 500+ and an Amiga 600 beyond the obvious change in chassis and PCB assembly techniques (600s are SMD, 500s are not) is the drive controllers - 600s, like 1200s, have integral IDE and a PCMCIA II slot.
To upgrade an existing 500 to 600 specs, you'd start with a 500+ so you already have ECS and KS2.0, then you'd need to add a hard disk drive controller of some kind - the official model is the Amiga A590. These are like hen's teeth in complete condition, and use either 50-pin SCSI drives or 8-bit IDE drives... no good for your CF requirements, so you'd need to keep looking. What are you wanting to run on it that needs to run from a CF card?
Just the local Commodore hobo and middle-aged PC hoarder.
eisa on Trademe. A lasting reminder of a Compaq fetish when I was younger.