Why does Luton need to change from a cheap-and-cheerful airport to more upmarket ? It's already loaded with debt from the Dart and doesn't need any more debt until its finances become healthier.
Luton is severely constrained on space - there would need to be a new terminal for this to change substantially. The new pier gates have substantially helped but Luton is likely to have an overcrowded feel for the forseeable future. The new shopping mall a few years ago has given it a more bling feel, but places like the champagne bar did not last long.
Yes, one could put in a false ceiling to hide things like ventilation ducts at certain locations and do other cosmetic changes, but it still won't address the overcrowded feel. Compare the terminal at Stansted or Gatwick North to Luton and you will likely find significantly more square feet
It's very easy to demand capital expenditure... but I just don't see a business case for it until the core problem is solved.