For upgrades you do not need to know the letters as it‘s usually already determined what letter upgrades get put inot (on LX usually C in business, but keep in mind that at airport upgrades you won‘t receive miles of the new booking class). Anyway to fresh me up and just so you know these are all the booking classes on swiss: (no guarantee for correctness, if something is wrong let me know)

F: Full Fare First Class
A: Discounted Fare First Class, RTW books into this class
O: First Class Award

J: Full Fare unrestricted Business Class
C: Full Fare nearly unrestricted Business Class (also used for opups and bought upgrades at the airport to my knowledge)
D: Business Fare
Z: Discounted Business Fare, RTW books into this class (LH books RTW into D!)
P: Heavily Discounted Business Fare
I: Business Class Award
R: Non-Rev Business, not sure if still in use

Y: Full Fare unrestricted Economy Class
B: Almost Full Fare minimal restricted Economy Class

M: Almost Full Economy Fare with few restrictions
U: Higher Level Economy Fare with few restrictions
H: Economy Fare with moderate restrictions
G: Economy Fare, some restrictions, used for group travel?

Q: Discount Economy Fare with moderate restrictions, used for RTW trips
V: Discount Fare Economy, often with a lot of restrictions
W: Discount Fare Economy often with many restrictions
S: Ready-To-Fly Economy Fare

T: Heavy Discount Fare Economy Class with a lot of restrictions
L: Heavy Discount Economy Fare with lots of restrictions
K: Heavy Discount Economy Fare with lots of restrictions
E: Lowest Economy Fare with lots of restrictions

X: Economy Class Award
N: Non-Rev Economy, not sure if still in use