Hi , i want to understand how eupgrade is used up, have an upcoming flight YUL-YVR-HKG (i do not think i have any chance clear to J YVR-HKG, but gotta try anyway), if i am upgraded J in YUL-YVR and then PY in YVR-HKG, what does the eup used and fee calculated. i purchased comfort fare.

i have limited eup and have just enough for 2 x J and paid the fee, or 4 x PY on this upcoming flight for both of us

i tried reading the wiki but not clear to me

Thanks in advance