When you trade a weapon with for example +4 for another in even trade the other weapon you get is also +4 but i dont think the +4 will stay if you upgrade to a higher weapon with AA, i'm not sure but i dont think it stays
for example;¨
Staff Of Life +4 gets traded in for Elven Long Sword +4, no problem
Stormbringer +4 most likely cant be upgraded to Sword Of Revolution +4