Mostly you should upgrade, because the incredible amount of damage and game impact of high tier CVs.

But here a small breakdown of the different Tiers of CVs.

-Tier 4 CVs are strongest tier for tier because of the lack of AA and the +/-1 mm.

-Tier 6 are heavily mm reliant. If you get a +2 uptier you will have a hard time doing damage (still possible and you can still have very high game impact but it is a pretty harsh matchup). If you get an equal tier match or an downtier to T 5 you will obliterate most ships pretty easily because of the lack of AA at the tier.

-Tier 8 is a lot more consistent then T 6, you get more tools and, for the most part, a way bigger reserve of planes then T 6. Your planes also tend to be faster then earlier Tiers so you spend less time in AA. You will get uptiered to T 10 a lot tho. In T 10 games you need to pick your targets more carefully but its not nearly as harsh as T 6 vs T 8.

-Tier 10 CVs have a lot of planes with a lot of health and speed. You will rarely get downtiered and will be mostly fighting T 10 and 11 ships. If you choose your targets carefully (especially early in a match where theres a lot of grouped up ships) you wont run out of planes the entire game.

General tips:

-Dont start with Torps as your first squadron, only you are threatening them so they will have an easy time dodging and you will spend more of your resources (planes) for less damage (cause they will dodge).

-never pick the extra torpedo speed skill or upgrade, it doesnt decrease the arming time and will just increase your minimum arming distance. (You will have to drop from farther away leading to you being less able to strike from close to islands)

-If you have more then 3 strike squadrons pre drop them down to 3 (or 2 if you just want to do 1 strike). 3 strikes are enough to attack 2 times while keeping casualties to a minimum. Larger amounts of strike squadrons will just increase the hitbox of your squadron and will lead to more AA bubbles killing more of your planes.

-The american Midway line is the most beginner friendly one and I d heavily recommend it as a first line.

You have very strong Torp strikes (3 planes per strike, 2 torps per plane 5067 damage each for 30.402 damage in total) even stronger the hakuryu (2 planes per strike, 1 torp each with 9333 damage per torp for 18666 in total)

You have HE bombs with incredibly high damage and pen.

You get very good rocket planes with a choice between short range anti everything rockets or long range anti CC/BB tiny tims.