Most modern heat pump systems account for that and have electric heating/defrosting built-in for those few cold days a year.

Also it's not like you have to remove your old heating system*; you can just plumb in the heat pump into your central heating and have cut off valves for the original system, so you can still use it as a backup.

*) unless your government is retarded and in order to get subsidies they require you to uninstall it