# Changing the elements of the list based on subscripts

Say I have a list of lists:

{{Subscript[a,1],Subscript[a,1]+Subscript[a,2],Subscript[a,-3]},
{Subscript[a,1],Subscript[a,-2],Subscript[a,-4]}}


Now, in the sublists of that list, if an element has a negative subscript I want to change this element to a negative of that element with a positive subscript, so if I applied this to the list above I'd get:

{{Subscript[a,1],Subscript[a,1]+Subscript[a,2],-Subscript[a,3]},
{Subscript[a,1],-Subscript[a,2],-Subscript[a,4]}}


I don't really know how to do it. Any help would be much appreciated.

expr = {{Subscript[a, 1], Subscript[a, 1] + Subscript[a, 2],    Subscript[a, -3]}, {Subscript[a, 1], Subscript[a, -2], Subscript[a, -4]}}