No, a player cannot intentionally discard down to 9 coins, resources, or cards during their turn to prepare for the next round. The rulebook specifies that players must discard down to 9 coins, 9 cards, and 9 resource tokens at the end of their turn (see Page 14 and Page 11). There is no mention of players being allowed to discard items voluntarily during their turn outside of this end-of-turn requirement.

Additionally, the rulebook states that if a player gains items exceeding their capacity limit during another player's turn, they do not need to discard down until the end of their next turn (Page 11). This reinforces that discarding excess items is strictly tied to the end-of-turn phase.

Summary

Players are required to discard down to 9 coins, 9 cards, and 9 resources only at the end of their turn. There is no rule allowing voluntary discarding during the turn itself.

Source: Wyrmspan Rulebook (pp. 10, 11, 14, 5, 6)