Resolve the sequence before installation
Ask the electrician and heater installer whether the existing supply can legally and safely serve the selected model until the upgrade. Request the conditions, permitted duration, inspection needs, and any work needed before power is applied.
Do not substitute an extension cord, borrowed circuit, adapter, or altered internal wiring for that design. A provider should identify an approved solution rather than leave you to connect the heater yourself.
If the existing heater remains in service, have its responsible provider assess whether that is appropriate and what happens if it fails before the upgrade. Do not assume keeping an old heater automatically makes a workable waiting plan.
Name the work and payer
Put temporary electrical work, permanent work, inspections, changeover, and removal of temporary equipment in writing. Identify which provider performs each step, who coordinates scheduling, and who pays for repeated visits or reconnection.
Ask whether the heater will be installed but kept unpowered, installed later, or supported by another approved arrangement. The quote should explain protection of equipment and the prerequisites for startup.
Plan household needs
If safe hot water will be unavailable, arrange another location for essential bathing or washing. Tell the coordinator about access needs and scheduling constraints so the interruption plan is realistic.
Request final electrical and heater startup verification after the permanent work is complete. A temporary successful run is not proof that the final installation has been checked.
Your next step is to request one written sequence from the electrician and heater installer. Do not energize the new equipment until its supply and startup prerequisites have been confirmed.