How to decribe an action with STRIPS?
An instance of propositional STRIPS planning is a quadruple $\langle P,O,I,G \rangle$


$P$ is the set of atomic ground formulas;
$O$ is the set of operators (actions); an operator  $a \in O$ has the form $Pre \Rightarrow Post$, where $Pre$ is a satisfiable conjuction of positive ($a^+$) and negative preconditions ($a^-$) of the operator; $Post$ is a satisfiable conjunction of positive ($a_+$) and negative ($a_-$) postconditions of the operator ($a_+$ is called the add list, $a_-$ is called the delete list);
$I \subseteq P$ is the initial state;
$G = \langle G_+, G_- \rangle$, called the goals, is a satisfiable conjunction of positive and negative conjunctions;
the state $S$ is a subset of $P$.


The result of applying operator (action) $a$ on state $S$ is:


$(S \cup a_+) \setminus a_-\quad$ if $a^+ \subseteq S$ and $S \cap a^- = \emptyset$
$S$ otherwise


In your example simply set: 
$$a^+ = \{q\}, \quad a^- = \{p\}$$
$$a_+ = \{p\}, \quad a_- = \{q,r\}$$ 

For a formal definition and a nice computational complexity characterizations of STRIPS with various constraints on the preconditions/postconditions see:
The Computational Complexity of Propositional STRIPS Planning (1994), by Tom Bylander.