Changing key can be easy or hard…
… and if you want it to be hard, then you just need to pick two keys that are more distant (i.e. that have less notes in common).
Why should you do that? Well, modulations between distant keys are more dramatic and more ‘modern’. I mean, not all of us want to sound like Bach, do we?
How far can we go?
Well, we can try and take two keys that are as far apart as possible.
these would be two keys one tritone away (like C major and F# major)
Then we can try to modulate between them.
As usual, we can do this in an abrupt/dramatic way, or in a smooth way – depending on what works for the song.
In this video we see how to change key between two keys a tritone apart, in several different ways – which one do you like best?
There are more ways to modulate between keys far apart.
For instance here we see how to use diminished 7th chords to move to distant keys:
And here we see other possible strategies for keys that are a major third apart. The examples I make here are between Am and C#m… a very interesting and underused key change