If MP3 is lossy and M4A is lossless (e.g. MP3 → WAV), the M4A file is no better than the MP3 — you can't recover information that's already been thrown away. If MP3 is lossless and M4A is lossy, expect the M4A codec to recompress; at 192 kbps this is transparent for most content.
By default yes (48 kHz MP3 → 48 kHz M4A). If you need to downsample for compatibility (e.g. 96 kHz → 44.1 kHz for CD burning) the advanced sample-rate option does this with high-quality resampling.