Can the existing ducts be reused?
Sometimes. Ducts are part of the system design, so reuse depends on their physical condition, insulation, and whether the zoning still matches how the home is used. Reuse can save cost and disruption, but it can also preserve the exact problems the replacement was meant to fix. Hyde inspects the ducts before saying either way. Read can existing ducts be reused in a ducted replacement for what has to be true before reuse is a good idea.
Timing the replacement
If the heater is ageing but still running, replacing it outside winter avoids being without heat and avoids waiting in the queue when everyone else's heater fails. A straightforward gas swap that reuses the ducts is usually a one-day job, while a full reverse-cycle changeover with new ductwork is typically staged over more than one day. If the heater has already stopped, start with urgent heater repairs and talk through the replacement afterwards.