Yes, the items you currently have will remain their level 50. So that’s why you will get better items when hero level grows and you eventually will need to replace those items by items with much better statistics and higher numbers.
If you don’t exchange them your raids will become very difficult. For getting more troops, you need higher leadership, to let the king survive at harder bases you need more health and better protection and so on.
So when your hero level grows to for example to level 50 you will be able to find items in uber chests up to level 80 and in other source of items (granny or non uber chests) up to level 70.
The higher the rarity number, the better the stats. When you get items with much better stats, you usually exchange your current items, unless your current item has something special where you benefit a lot from (like skull perk, farm perk, start morale, high leadership, e.t.c.).
Remember that items will become outdated eventually and need to be replaced. When you reach level 100+ you will profit longer from those items and don’t need to replace them that often. At lower levels however replacement is a must, otherwhise you fall behind and suffer during raids. Good items will help you in offense, outdated ones will do the opposite.