It can be said that we see Friar Laurence change with respect to how he acts upon his principles.
For instance, it is not clear that he truly believes that Romeo and Juliet genuinely love each other. When Romeo first tells Friar Laurence of his love for Juliet, Friar Laurence declares, "young men's love then lies not truly in their hearts, but in their eyes" (Act 2, Scene 3). However, for moral and political reasons, he agrees to marry the couple immediately, because he believes that their marriage may put an end to the war between their families. On the one hand, Friar Laurence is upholding his moral responsibility to ensure that his parishioners are peaceful and loving towards each other, but he is doing it at the expense of making a holy marriage he feels is ill-judged and ill-timed, thereby letting his moral responsibilities towards performing marriages slide. In the scene where Romeo and Juliet are together in his cell, just before he marries them, he reminds us that he thinks they are being too hasty when he tells Romeo, "Wisely and slow; they stumble that run fast" (Act 2, Scene 6). He even preaches to the couple that he feels their violent, passionate love is unwise when he says, "These violent delights have violent ends and in their triumph die, like fire and powder." In other words, he believes that their violent, passionate love is doomed to fail, but agrees to marry them any way, and all for the sake of trying to unite their families.
However, though he makes it evident that he feels the couple's marriage is foolish and could end violently, he does not council Juliet with the same wisdom when she comes to him for a way to get out of her betrothal to Paris. Instead of continuing to preach to Juliet that their hasty marriage was unwise and doomed to failure, he immediately agrees to help her and devises the plan to fake her death.
Hence, throughout the play, we constantly see Friar Laurence change with respect to deciding which moral obligations he should uphold and which he should drop.