Can upconverting 1080p videos on 4k TVs, lower their quality?
Upsample by an integer ratio can be completely blameless, just repeating pixels and lines gets it done, no new information, and you get only the original resolution displayed, but no losses either. 

This zero order hold based approach tends however not to be popular, and the amount of processing to do anything else rises rapidly especially if you either want to apply filtering over more then a very localised area or play dynamic games to try to give the impression of more resolution (Popular with consumer gear which compounds the messing about by not having sufficient processing to do it well).

Just doing a 2D sinc interpolation is pretty close to blameless (potentially 90% bandwidth or so if you can afford to throw sufficient processing at it), and takes the edges off the pixel doubling.

However for most reasonably sized tellies at reasonable viewing distances, HD is in fact just fine, 4k is nice on a computer monitor where you are sat 18 inches away from a 30 inch screen, but that is not the usual use case for telly. 
You may find a search on the term "Circles of confusion" to be informative.

Now HDR (On a set that has the brightness and contrast ratio to pull it off, not technically easy) is a real win even on a SD screen.