well, most of what can be said about how to change keys within a song has been said, but for good practice try these two pieces...

"Blue Bossa"- (suggested version by Pat Martino), which has a moderate but noticable key change at the start of the ninth measure of the melody

"I Love Paris"- (suggested version by Louis Armstront), which has a very noticable key change at the start of the twenty-fifth measure of the melody

for more practice, i would suggest getting "The Real Book", quite possibly the greatest collection of jazz pieces ever culminated and transcribed for guitar... Blue Bossa and I Love Paris are both in there, along with many others that have significant key changes to check out