retraining class question

question1 can I retrain a class to its own?
As retrain a level 1 oracle to another level 1 oracle.
question2 if I can retrain skills by retraining the class level, why shall I retraining skill
one rank by one rank given my int is 10

In general, it takes 7 days to retrain one level in a class into one level in another class.

(emphasis mine)

You can only retrain the class level into a totally different class. If you want to do oracle->oracle you'd need to retrain the level into something else (probably a synergy class so it doesn't take as long), and then retrain it again back into oracle. This will take 10 days, so if you are only planning on retraining up to your int mod skill points, it's more cost and time effective to simply retrain the skills.