If you select a step in the proof and give the Rejustify command, then the step and its justification will be deleted, and they will be replaced by a gap whose goal is to prove the assertion.

Note that if the step is justified by a subproof, you can use the
Rejustify command even if the subproof still has gaps in it. For example,
suppose you are proving a statement of the form *P**Q*, and you begin by using the Direct command. This would create a subproof in
which it is assumed that *P* is true, and you must prove that *Q*
is true. The subproof would justify the assertion that *P**Q* is true. While trying to prove
*Q* in the subproof, you might run into trouble and decide that you
would have been better off proving *P**Q* by proving the contrapositive. If so, click on the
assertion *P**Q*, which will
select the assertion and the subproof that justifies it, and give the
Rejustify command. The assertion and subproof will disappear, and be
replaced by a gap whose goal is *P**Q*. Now you can select the goal and give the Contrapositive command.