Short version: you're fine with your proven 649; throw a better front sight on it and be done.
Longer version...
There's no way of knowing if S&W had a bad run on 640 Pros without access to S&W's internal records; otherwise it's speculation based on forum talk with by-and-large only the disappointments accounted for -- folks don't often take the time to simply post "all good here".
My guess is that like most firearms from any established manufacturer there are more 640 Pros built right than there are lemons, but I have seen the same complaints on different forums that you have and think there seems to be a few more on this model than average.
My own experience of the single example I've owned is that overall it was well put together and I've had no issues regarding parts and reliability. It shoots low with virtually anything I put through it, a complaint common among many though not all 640 Pro owners; it does seem to be a drive the dot sight set-up which accounts for some of this, but I think S&W got sight heights slightly off here and there's no replacement as yet from the factory or aftermarket that I'm aware of. Not the biggest deal -- you learn your gun and where it shoots using the sights its got and the ammo you prefer, and proceed accordingly, but I do wish the piece showed greater flexibility with different rounds; my 442 runs 110s through 158s, plinkers or defense, much closer to point of aim than the 640 Pro does.
Mine did arrive from the factory with a ridiculous barrel-cylinder gap, which they corrected on their dime. The 442 did, too.
The trigger arrived very smooth but heavy and spongy and has remained that way, whereas the 442 is also smooth but slightly lighter and has the crisper mechanical feedback from the lockwork I'm used to from a S&W revolver. Both work, though.
I think the 640 Pro is S&W's best concept for a J-frame defense piece, and if you're significantly drawn to one I'd say get it, you're likely to do well, and if there are factory caused issues, S&W will deal with them.
That said, I don't think the 640 Pro offers anything of substantial value over your 649 to justify the cost and effort of switching; replace your 649's front sight with an XS standard or big dot and you've equaled the only significant advantage I think the 640 Pro has. I love moon clips, but most seem not to and they're by no means a dealbreaker to me, and after that, what else? A fluted barrel? Useless aesthetics on a pocket snubby. No lock? I prefer no lock, but it isn't to me a reason to switch if I already have a 649 that has proven itself.
You're good as you are methinks, and money usually better spent on ammo and training. But if the 640 Pro won't stop calling, there's no reason to fear answering.
Now, waiting for Edmo to chime in...
