Here's a function that converts using interlaced=true without the need to change it for an Avisynth filter that's probably had the same default for 20 years.
Code:
function KatieRGB(clip Source) { ConvertToRGB(Source, Interlaced=true) }
There's a function on the Avisynth wiki for converting "mixed" content, although I can't say I've used it myself as the first filter I'd add to a script for an interlaced source would be a deinterlacer.
Code:
function KatieToRGB(clip Source, int "threshold", bool "debug")
{
debug = default(debug, false)
global threshold = default(threshold, 20)
Vid1 = ConvertToRGB(Source, interlaced=false)
Vid2 = ConvertToRGB(Source, interlaced=true)
ConditionalFilter(Source, Vid1, Vid2, "IsCombed(threshold)", "equals", "true", show=debug)
}