How can I transform a mapped BAM file into an unmapped BAM file? In order to use MergeBamAlignment (Picard), I need an unmapped BAM file and a mapped BAM file. I have two mapped BAM files: 


one with reads mapped to the reference I want but without metadata such as read group
a second one with reads mapped to the incorrect reference but with metadata


When I try to use MergeBamAlignment with these two files, I get errors, such as "Second read from pair not found in unmapped bam". So I am trying to unmap the reads from my second BAM file. How can I do that? Is there a tool for it? Otherwise, I'll have to manually change the bitwise FLAGs, reference names, CIGARS, etc.
Looks like you need Picard's RevertSam operation (described in context here, manual here).

java -jar picard.jar RevertSam \
     I=input.bam \
     O=reverted.bam