A week ago I would have talked about how easy it was with the chart of hp and damage.
Now I'm looking at converting Gardmore Abbey to 5e, and the beholder is the wrong level and almost no monsters are perfect, and I kinda miss being able to just add +/-1 to their defences and tweaking damage.
Especially since the MM doesn't line up with the expected numbers in the DMG in the slightest...
A quick formula for changing hp and damage would be nice. I'm sure someone could figure it out.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.