About generals :
- There are 8 slots, 4 for offense boosting items, and 4 for defense boosting items.
All items are 100% specialized in only one way. i.e. you won't find any shield boosting offense, nor will find any weapon boosting defense.
So "switchng" is totally nonsensical here.
Just collect items and replace them when you collect a more powerful one, that's all.
Break the weakest ones to enhance the better ones.
- The most interesting items are those that cover all 4 categories : light + heavy infantry + phalanx + cavalry.
There are no "phalanx only" nor "cavalry only" etc. items, meaning any item covering cavalry will inevitably cover the 3 other categories as well, with the exact same value (as values are either 0, or all the same anyway).
These are class IV items and are the only ones you should try to get, and that are worth upgrading (though the upgrade costs are very high, but since you can break all the rest, you should get enough pieces sooner or later). :p
- Of course, you may find some uncommon, rare or legendary items that yield better characteristics, but are of lower class than your, for instance, common class IV item.
Here you'll have to decide for a strategy : either keep the rare item giving you better boosts, but for/against low categories units only, or keep the universal one and enhance it to get better boosts in the end.
Well, considering what I said above, you can't count on some other rare item you could put in another slot to cover the phalanx and cavalry boosts you're missing, since it will also cover the lower ones anyway, so some combination of complementary equipment is not conceivable.
Yet, replacing any low class items by higher class ones is still a better choice IMHO, unless you're for instance using mostly offense infantry, then in this case, wearing a low class but rare or legendary item is probably a much better choice (and is cheaper to upgrade).
For this reason, I sometimes keep some good, but low class item spare, in order to switch when needed.
- Note that what can be true for offense, where you're the one choosing what units will fight, can probably not be for defense, when you don't master what will attack you.
I think in the case of defense slots, universal (class IV) items should stay a preferable choice in all cases (in order to keep no 0% bonus against higher class offenders), but once again, it's up to you. :)
As of course, if you know your opponent will usually sent you a buch of hoplites, then equipping an upgraded legendary class I item with a high defense bonus will be more efficient than equipping a poor class IV one. ^^
- About chests : their contents is random, but their class and quality are known in advance. It's clearly written in their description (and anyway, the needed key tells it all), so you know if it's worth buying a key (in case you don't have earned any matching one) or not.
I currently have an unopened legendary chest, but I'm not interested into buying a key, as it's only a class II one, and I'm all equipped in class IV already and don't wish to lower my phalanx and cavalry attributes. However, I'd be interested into breaking it for pieces, so if I can get a matching key by any means, I'll open it. Else I'll just drop it, as I consider it moslty useless.
HTH. :)