[pawel]

MPG (DVD) is lossy thus when you convert it to AVI and back you will use original quality - level depends on what software and setting is used.

Re-synching doesn't require such conversion you mention because what you need is to demux (separate) video and audio stream. Then audio can be replaced/mastered/whatever in a dedicated prog. However, most MPGs contain AC3 (Dolby Digital) audio format which is lossy. There is no any editor which works directly on AC3 format, and input file is converted to WAV or AIFF (and back to AC3, lossy).

If the source material is good and conversion is done properly, it's hard to recognise quality loss. Anyway, mastering of native lossy files should always be specified in the lineage.