I am concerned in that our viewing sources cover the range from 480p to 1080. Most are 720.
I have a 5 year old Samsung and the videos are descent from all sources.
Looking at the Samsung Q80t, with the higher resolution will it handle the lower sources,
I don't want to loose what we have. I realize that a 720 source won't be as good as a 4K, but will it be as good as I currently have with the older Samsung ?
Considering the screen size of your old TV and the new TV are the same, the visual experience will not change from what you currently have. If you were moving up to a 77" TV then the 720 upscaling would not look as good due to pixel density. I hope that makes sense. The upscaling is done by the TV itself and the newer processors do a very good job. In fact, the newer models may provide slightly better upscaling as the tech is improved.
To be clear, the Samsung Q80T is still an LCD display. It's a very nice picture but it's not an OLED display. The LCD's tend to be brighter but the blacks suffer due to the backlighting required, this leaves blacks looking more like shades of very dark grey. OLED's don't use backlights so the blacks are truly black, inky true black. So this true black helps to improve the contrast.
I feel ya man, I have tons of old movie DVD's and many are 720p.
The AVS forums will have more info if you want to check them out regarding your questions.