How far are you from the screen ? Viewing distance will play a big part in how SD looks on a larger screen.
I moved from a 46" 1080p Panasonic plasma to a 55" 4K Panasonic OLED and SD from around 8-9ft looks fairly similar on both - ie. poor quality SD sources still look poor and better quality SD sources look passable, when there is no alternative. On some occasions, I'm actually quite surprised just how good the better quality SD content can look on the OLED,
considering how very little picture information it has to work with so, with that in mind, I would consider the upscaling abilities of my Panasonic (GZ) OLED to be good.
While an SD picture will inevitably look 'softer' on a higher resolution 4K screen, better contrast (especially if you're moving from LCD to OLED), colour, motion handling and other factors may all help to give an SD picture a boost to offset the loss of sharpness.
1080p HD looks pretty similar on both from my viewing distance (in terms of sharpness) so if you mostly watch in HD, then upgrading from 50" to a 55" 4K screen might be worth it for the larger picture but if SD makes up a good proportion of your viewing and you like the screen you currently have, then I don't think it's worth upgrading for a slightly bigger picture with the inevitable drop in sharpness and upgrading from a 50" 1080p screen to a 50" 4K screen would be completely pointless IMO, if you have no intention of viewing 4K content.
I used to watch quite a lot of SD Freeview but, since upgrading, my viewing habits have changed and I now watch almost exclusively in HD and 4K (Freeview, Netflix and Prime) and very rarely watch anything in SD. Once you've watched something in 4K Dolby Vision HDR on an OLED, you'll struggle to go back to SD Freeview !