When is a conviction an agg for sentencing purposes

A plea of guilty is a conviction for prior record level, regardless of whether a judgment of punishment is entered.  That is why a Prayer for Judgment Continued is still a prior conviction for sentencing.  When someone is placed on 90-96 probation, they enter a plea of guilty before being placed on the probation; but, as you said, there is no adjudication of guilt at that time.  However, State v. Hasty, 133 N.C. App. 563, 570 (N.C. Ct. App. 1999) says that a sentence for a new offense, imposed while on 90-96 probation, is properly enhanced by the offense underlying the 90-96 probation because the defendant violated the 90-96 probation when they committed the new crime.