Family First Transition Act This bill provides additional funding for, and otherwise modifies, implementation of the Family First Prevention Services Act, which provides state, tribal, and territorial child welfare agencies with certain spending flexibilities to support substance abuse services, mental health services, and other services that prevent children's entry into foster care. Specifically, the bill provides one-time funding to jurisdictions to reduce adverse fiscal impacts associated with transitioning funding sources in relation to the new spending flexibilities. The bill also provides additional funding for FY2020 and FY2021 for jurisdictions with certain waivers (currently set to expire at the end of FY2019) that allow them to provide services not authorized under the traditional foster care program. In addition, the bill temporarily delays a requirement that at least 50% of certain funds be spent on programs and services that meet the criteria for well-supported practices. For FY2020 and FY2021, this requirement shall not apply, and for FY2022 and FY2023, programs and services that meet the criteria for supported practices shall be considered well-supported practices.