What's happens to credit in the non-Organizer account when you turn on Family Sharing?
Yes. Gift balances are preserved.

In fact, I use a gift balance to avoid charging the family organizer for my primary Apple ID. I want to pay for my apps and be part of the family to share apps and calendars and reminders easily.

This lets me use my Credit Card to buy gift cards (usually at a substantial discount from Costco) and/or use Apple Pay and Passbook to load my store credit at Apple Retail Stores without needing a physical gift card at all.