Watch the Dealing with Chord Changes online guitar lesson by Fareed Haque from Soul Jazz Survival Guide
So this is why it's so important to practice those blues scales in all positions. Try practicing chords in pairs at first. So, for example, practice D7 to G7 in one position, then G7 to C7 in one position, then C7 to F7 in one position, etc. Then put 'em all together.