The changes you've referred to are developments of doctrine and not breaches with basic principles. Doctrine does indeed develop but always stays true to its fundamental premises. 

"Some advocate that learned truth can change old guidelines; others insist the old guidelines are by definition perfect."

Doctrines are not "guidelines" and that's a false dichotomy.  No one believes that doctrines on faith and morals are "perfect". There is always room for improvement and for God to reveal more to His Church. However, they are indefectible and infallible which means any development has to be consistent with existing teaching.

So far as the teaching of Jesus is concerned, it really requires no explanation and isn't open to different interpretations.