Name: Rolf Mertig (email_not_shown)
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]

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