Great question. Having the good fortune of my kid rising to the occasion in his first year and being new to HS ball and CIF rules, I've been seeking this same information without having to ask the coach and have him come away thinking my son is not thankful for the opportunities provided so far, or that we want him to be on Varsity just to say he made the team. IMHO, innings and AB's are of utmost importance at this level.
Anyways, I looked it up last night and believe that Rule 302-6 dictates that a player can play between all levels during tournaments, but once rostered to Varsity (I would consider "rostered" to mean the roster submitted to the Section Office as mentioned in the rule below) cannot move down. The question I came away with is the due date for eligibility rosters to be submitted to the Section Office. I'm hoping the rosters are "due" just before Conference play begins and that my kid will continue to face Varsity and JV competition until rosters are submitted, then being rostered to JV where he will likely get double the innings and AB's during the season than he would see on Varsity.
Here is the rule cited from what appears to be a CIF website for LA/OC:
302-6 Other than for tournament play, movement between Frosh/Soph, JV and Varsity level teams is allowed up to the date that the eligibility rosters are due in the Athletics Office. Students who are moved to the varsity level cannot return to the lower level after the date that the eligibility rosters are due in the Section Office. Although movement up to the varsity level is allowed at any time during the season (with adherence to 301-3 above); no student shall participate in more than one level of competition in the same sport in the same day. Exception: Those transfer students who have been restricted to lower-level may not play on the varsity level throughout the duration of the restriction.
For tournament play, students listed on the junior varsity or frosh/soph roster may participate in a junior varsity or varsity level tournament and maintain their lower-level status throughout the remainder of the season. However, no student listed on a varsity roster or junior varsity roster, may participate in a lower-level tournament at any time during the season. Exception: Those transfer students who have been restricted to lower-level may not play on the varsity level throughout the duration of the restriction.
My response is just an inexperienced assumption, so any further clarification will be greatly appreciated.
Best of luck to your kid this year!