There are sort of two ways you can look at modulation (switching between keys). You can see it from a utilitarian standpoint (“just get ‘er done”) or you can try to get artistic.
Let’s face it. Unless you are a great musician with a lot of experience, most modulations are going to fall into the “just get ‘er done” bucket. Most of mine certainly do. That is because in real life, most modulations are largely unrehearsed and unplanned. I never spend any time thinking about how I am going to transition from song to song in church. I just take note of the two keys when it is time to modulate and do something to make it happen. I am not saying this is optimal. I am just saying that is how it tends to happen.
If you are going to get artistic, you tend to think a bit less in terms of modulation formulas and such and start thinking about ways to create an effect with the modulation itself. Here is an example of something I am doing now with a song on my new project.
We are going from the key of Bb to C and there are several approaches we could take. We could use a formula like I – vi in the old key to ii – Vsus – V7 – I in the new key. That is the formula I learned in college. It works but if you are not careful, it just becomes a utility rather than art.
We could also look for pivot chords (chords that exist in both keys). The most obvious pivot chord is Dmin. Dmin is the iii chord in Bb and the ii chord in C. It is the simplest choice by far because getting to the ii chord in the new key makes things really easy. You just have to move from ii to V to I and you are good to go.
Assuming we are going to use the pivot chord of Dmin, the tricky thing involves how you get to that chord to start the mod into the new key. Getting to the pivot chord is always the most delicate part of a mod.
So here is what I am playing in this particular situation.
Note that this is an 8-bar modulation but it is really one 2-bar progression repeated 4 times in a row:
Gm7 – Gm7/F – EbM7 – D7(alt)
In order to build into the next section (which is huge), I am getting busier and louder on each progression.
The idea is to get the listener expecting something and then throw them a little twist. After playing this progression with a D7(alt) on the end three times in a row, that is what the listener is expecting on the last progression. But instead of going to D7, I go to Dmin7 which is my pivot chord. I then go straight to G7 (V7 in the new key) and I am ready to start the next section.
As you can see, the modulation technically takes place completely in bar 58 but in my mind, it really starts in bar 51. I am just taking 7.5 bars to set up the pivot chord the way I want to.
Can you do this kind of thing on the fly in church? Probably not. But as you arrange, look for ways to make something of your modulations rather than just finding a chord formula that works.