[FFmpeg-user] Splicing together videos with different characteristics

[Full Name]

I have a series of videos of my daughter giving piano recitals over the years. The videos start when she was very little, playing with a single finger, till now, that she can play the piano with high proficiency. I would like to splice them together into a single video.

The problem is that the first few videos were obtained with an older camcorder, that could do 640x480 resolution, whereas the later ones were captured with a newer camcorder, able to do 1920x1080 resolution. At some point I used a combination of HandBrake and ffmpeg to splice them together, but with incomplete success, in that the aspect ratio of the 1920x1080 videos was modified to match that of the 640x480 ones. The end result was, of course, very unsatisfactory.

I was wondering if ffmpeg might allow me to obtain better results. Is at all possible, with ffmpeg, to splice such videos together, so that the correct aspect ration is maintained for all the portions in the final video? I would have thought that the answer is 'no', but my knowledge of these things is very shallow. 

More in general, what I would like from experts in this forum is advice on how to splice together the videos above, in order to get the best results, using ffmpeg.