Only a word of caution -- in Hebrews (the NT letter to the...) it is noted
that when Abraham payed tithes to Melchizedek, Israel / Aaron? paid tithes at
that time (being still in the loins of Abraham) -- paraphrased from memory with
apologies. I only note this because there wouldn't be a question of
pre-existence in Hebrews regarding the Aaronic priesthood in the loins of
Abraham so much as ancestry -- yet, spoken in terms that some might construe as
pre-existence if they were so inclined (on much better grounds than this short
snippet in John you allude to). Ok, I didn't mean by this to answer your
question in lieu of an authority -- I just wanted to note that your question
provoked this cautionary flag in me when I read it. Hope it may be of some
help (right or wrong).
Hebrew Union College - JIR
A word is not a crystal, transparent and unchanging;
it is the skin of living thought and changes from day to day
as does the air around us. -- Oliver Wendell Holmes.