Old Regime and New Regime

A senior citizen using ITR-4 for AY 2025-26 is considering switching from the old to the new tax regime due to a previous loss. They are asking if they can refile their AY 2024-25 return under the new regime and what form is required to opt for the new regime for AY 2025-26. They also inquire about future tax filing (ITR-1/ITR-2) and the possibility of switching back to the old regime later if beneficial.