I wish United reused old regos cuz now instead of using N###UA they use 5 numbers after N. American isn't much better, for their 737 MAX 8s they seem to be using three numbers after N and then two random letters (an example I flew on in December on ORD-MIA is rego'd N812UP). Delta is the only of the three kind of making an effort, using DA, DL, and DU after three numbers.