Theories of how language works have shifted from rule-like competence accounts to more skill-like incremental learning accounts. Under these, people acquire language incrementally, through practice, and may even lose it incrementally as they acquire competing mappings. Incremental learning implies that (1) a bilingual's abilities in their languages should depend on how much they practice each (not merely age of acquisition), and (2) using an L2 more could cause a bilingual to gradually 'unlearn' their L1. Using timed picture naming and vocabulary measures, we tracked 139 children for several years as they transitioned from mostly-Spanish homes to mostly-English schools. Following their increased English use, many became more proficient in English than Spanish around the third grade, demonstrating continual learning. But their Spanish also improved, showing that L1-attrition is not inevitable. Incremental learning explains both co-improvement and L1-attrition as consequences of experience-driven learning: improvement from continuing L1 use can offset competitive unlearning.