Some more thoughts on Modulation.....

If you want to change to a 'parallel key' such as C major to C minor, you can just switch, without other chords.
Another commonly used modulation method is to use a 'Pivot Chord'. The trick here is to end the tune/song in the first key, on a chord that also exists in the second key - this is the Pivot Chord. For example, the chord A minor exists in both C major (vi min) and G major (ii min), so it would be a good pivot chord. So, if you’re playing along in C, a usable progression would be C F G7 C > Amin - Amin being the pivot, you can then continue in the key of G - Amin D7 to G.

There is a good 'tutorial' here : http://www.lovemusiclovedance.com/modulation.htm