It'll be like Amber/Jadeite was at the start of Factions, once everyones got them it'll be sorted, admittedly I did buy quite a few Bloodstained ones as soon as I saw them appear as me and a friend both have 3 sets of NF Armour and theres a couple of new people I know and I try to help them out whenever possible, also saves wearing the old boots when we do MM, although you do see what the popular sets are per class judging by the fact that MM ones shot through the roof, just glad Tormentors and the other sets are still reasonably cheap and Undertakers is certainly handy for my Sac Build as my health goes up and down like a yoyo
Hope they do this for the FoW sets as well seeing as thats not exactly cheap to swap about if you decide to change your build, but mines a mixture I can use on all my builds anyway so I wont be upset if it doesnt get applied to the existing people with it, although it should if you have all 3 campaigns or NF with either Prop/Factions as all the Tyrian/Canthan Rune Traders have them in stock the way the storage upgrade applied if you owned Factions.
Havent tried it on my FoW so if they already apply then great (anyone tried?), if not it should

.