Name: Rolf Mertig Date: 05/02/17-10:27:18 PM Z

You could do something like this:
 
FCE@Calc[(LC[][a, b, c, p] + LC[][a, b, c, q]) /.
    p -> (pplusq - q)] /. pplusq -> (p + q)

LC[][a, b, c, p + q]