Yeahm they did that in the FL Keys when they put in all the new bridges.
Refering to the closeness, it's not an issue. They can put the chrges so that they blow up inward toward the old span, so not much flying material to damage new span. I'm sure they planned for it when designing the new bridge. Actually, speaking of FL, I was there a month ago and they didn't remove very much of the old bridges. They would go about 200 feet out and cut out a section between piers, so sailboats could get through but people couldn't get over. Then left up the rest of the bridge.
What they did in Charelston to get rid of old truss bridges: dynamite one span of the bridge at a time, after removing the concrete deck, and then go pick it out of the river (also shallow) with a crane and excavator. Then blow up the next span. see here for some pics and video:
http://oldcooperriverbridge.org/blast.php