Not much. The original 550a had the 'switch' as a small 'baseball bat' switch, the 550b has it integrated into the board and front panel. The 550b lost the pre EQ line out and replaced it with a separate pre EQ DI (nice) and added a mute switch. Sometime into the 550b production run, Dave changed the design of the preamp chip, but most could not hear much, if any difference.
So, mainly, a couple features (including a pre EQ DI, which is nice to have), and a bit more 'integrated'.
I was so into these amps at one time that I had the 'original 550' (which was called the 420 EU), one of the first 550a' (that TBer Burk is gigging now) and then finally the 550b (which was worth the upgrade to me for the DI). The all sounded virtually identical and they held their value nicely, making the 'upgrading' pretty painless over the years.
The 550 is one of those rare amps that sounds wonderful virtually flat (NEVER touched that timbre control, and maybe just a touch of enhance every once in a while... that control is much more subtle and usable than most of those 'scoop' controls, and even then, never ran the enhance past 9 o'clock), and really sounded good with literally every cab I paired it with (Epifani, Bergantino, Aguilar, LDS, whatever). With some cabs, a touch of enhance and maybe one click on the bass control, and you had a very warm, even, punchy tone. VERY nice!!!!
I have not tried the current model... a little too much for what I need, and the reported voicing change would not be in the direction that would be good for me personally. I bet it kills though.