Every crime is defined by two aspects: a material one (the actual action) and a mental one (the intention).
Applying to oral sex as such (not as foreplay) the elements are:
1. Eating pussy/cock
2. Aiming for an orgasm, immediate pleasure without having sex afterwards.
So, to answer your question, you don't "change" a crime buy doing something else, as oral sex as a foreplay lacks the moral element, and thus is not a crime.