Good advice. Changing keys requires knowing each key separately; else you learn by rote rather than by hearing the changes as they come.

A try (off the top of my head, at work with no instrument to check my accuracy).

I
There once was a union maid,
    IV          I
Who never was afraid
       IV
Of the goons and the ginks
       I
And the company finks,
         II7                V7
And the deputy sheriffs that made the raid.
    I
She went to the union hall
         IV             I
When a meeting it was called.
    IV             I
And when the company boys came 'round,
    V7               I
She always stood her ground.


Chorus:
         IV                                  I
Oh, you can't scare me, I'm sticking to the union;
                V7                      I
Sticking to the union, sticking to the union.
       IV                                  I
Oh, you can't scare me, I'm sticking to the union
                V7                   I
Sticking to the union, till the day I die.


In each key, the I is the #1 note/chord in the key, the IV is #4, and V is #5. Of course, II is #2, etc.

So in G,

I = G
IV = C
V7 = D7
II7 = A7

Apply to other keys as needed (watch out for the sharps & flats, though). This works best for C, G, D, A, & E, since the sharps & flats won't hit those notes (in E, II=F#, though).

Bob

(Now, when I post this, the spacing will be all gefrunzled, won't it? Sigh.)