Erin Moran indeed appeared on Family Affair long before she got the roll of Joanie on Happy Days, But there is a second link: one of Anissa Jones's best friends was Susan Neher, who also appeared on Family Affair. Later she had a show of her own To Rome With Love.
Most people don't know this but the pilot for Happy Days was an episode of Love American Style. And the actress portraying Joanie was Anissa's friend Susan Neher. She was replaced by Erin when the show went into regular production.

In memory of Anissa Jones:

http://www.geocities.com/bkohatl/AnissaJones.html