OK, I know someone at the inventory department.
This is what I've discovered for KL, from high to low.
(adjusted on 31Jan with more info):

Business cabin
J: usually most expensive business class fare
C: 2nd highest business class fare and the class you use when using a generic, all-airlines business class fare "full C fare"
I: new business class fare, usually (although not always) for corporate negotiated rates
Z: restricted business class fare or business class special, also: award upgrades from S-class
D: award tickets in business cabin, or award upgrades from B-class

Economy cabin:
S: full flexible economy fare
B: flexible economy fare
M: restricted economy fare = changes at a fee
K: very restricted or non-flexible economy fare
H: restricted economy fare = changes at a fee
L: non-flexible economy fare
Q: very restricted or non-flexible economy fare
T: non-flexible economy fare
E: non-flexible economy fare (european routes only)
N: non-flexible economy fare (european routes only) *
V: rock-bottom/action fare *
W: award tickets in Economy Cabin

* = earn 25% of flown miles

Flexibility can be set based on stay (minimum number of days or maximum number of days), change fees, cancellation policy, stopovers allowed or not, advance purchase periods and to allow the fare on certain flights or not.

Furthermore, the difference between certain fares like T and Q is not that great except in price. Thus, the fuller the flight is (expected), the sooner the cheaper classes close, forcing you to buy a higher fare. This allows airlines to get a mix of fares in. Or, for those who studied micro economics: to better skim the demand curve!!!

To answer your question: upgrades on miles only on S and B.
However, at a few stations like JNB and CPT, they allow you to, for a fixed rate per class, upgrade your fare to B and then spend miles to upgrade to Business Cabin, if seats available, on the day of departure.
However, not for V-clss and they told me in JNB this is a pilot restricted to a few stations, on KL only.

Hope this helps.