When to replace T17?

[Astynax]

All 3 of my characters have their T17 4 set. When would they replace their tier with other items?

Destro/Aff Warlock: 0 T18 yet, 2x of the T17s 706, 2x are 685s and can be replaced with 705s+.
Main raiding character and would have access to the most replacement potential, especially when others are looking for tier and nothing else I could pick up higher level non tier easier

Prot/Ret Pally: 4x 685 T17. Can soon replace with 2 695 T18 and a single 715
Not too sure how much value the RNG FV/DP procs are vs a guaranteed stat increase + triple wings. I think replacing now would be optimal especially for ST

Prot/Arms Warrior: 4x 685 T17. Can replace two slots with 695, one 700, one 715
Majority of the time run as prot. Decent set bonuses for shields but with the higher item levels I get more armor, which would be close to offsetting the bonus set damage reduction along with higher str & stam? This guy is my 2nd alt and last in line to get any sort of tier gear outside of bonus rolls in PUGs, but PUGing isnt too successful this early on

[Criptos]

You want to break 4pc T17 for destro as soon as you can make 2pc t18. Not sure about other classes

[EzG]

I'm not entirely sure this is the place for this. You could just split up those three items into three posts in the class forums and you may get better resulting responses.

[Rawrzillasor]

Prot/Ret Pally: 4x 685 T17. Can soon replace with 2 695 T18 and a single 715
Not too sure how much value the RNG FV/DP procs are vs a guaranteed stat increase + triple wings. I think replacing now would be optimal especially for ST

i would switch to t18 2piece for ret atleast the three wings is nice and its an ilvl increase even if using normal pieces vs heroic brf