1. Edit your video in its native HD interlaced 50i or 60i timeline
2. Be sure that all titles (if used) are placed on V tracks and not in T tracks, in fact I recommend to stop using T tracks for good.
3. When done editing the HD project, switch your project to SD and set resampling method to Lanczos 3
4. Then export to mpeg elementary streams for authoring in third party app, or use the built in DVD Creator, or if you need an SD AVI, export to Canopus DV and select MSDV option, this will provide a DV AVI file that will play in all PC and Mac. When the export is done, switch your project setting back to HD
Note: If exporting to SD DV AVI, be sure you select to export in 16 bit otherwise an illegal DV AVI could be produced if your project is 24 bit
Note: The above methods gives better downscaling results than exporting HD timeline to HD Canopus HQ and then loading the HD HQ file in SD project