I have heard rumours telling that the 318 will be certified with a new generation of control computer hardware. And when that has happened, then the 319/320/321 planes will very soon be certified with the same hardware.
It will probably be very much like upgrading from an Intel 486 to a Pentium III or such.
I also heard that when that has happened, then a fairly simple upgrade kit should be made available to fit on existing planes.
If that rumour is fairly precise, then of course the 340-500/600 will play the same pioneer role in relation to its present 330/340 sisters.
But it is no more than a rumour.
Major structural redesign should not be expected in the foreseeable future. Things don't change that fast. The 737 is no exception. Twenty years ago, when the 737-200 became the 737-300, then it mostly got a new engine, which it needed badly. The 737NG got a new wing after more than thirty years!
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs