Pick your appliance and roughly what year it was made.

Matches your inputs against EPA phase-out timelines
Look for a data plate or sticker — on AC condensers it's usually on the outdoor unit's side panel; on refrigerators it's typically on the back panel or inside the fresh-food compartment wall. It lists the refrigerant type and the charge amount in ounces or pounds.
No. Different refrigerants have different pressures and oil compatibility. Mixing them can damage the compressor and is not something a technician will do — a refrigerant swap means replacing the system.
A2L refrigerants like R-454B and R-32, and natural refrigerants like R-600a, are classified as "mildly flammable," not explosive. The charge sizes in home equipment are small and the risk is managed through technician training and equipment design — it's not a reason to avoid newer units.
No. The phase-out affects new manufacturing only. Existing R-410A systems can be serviced and refilled for the life of the unit — there's no requirement or deadline to replace a working system.