tcprewrite from the tcpreplay package can be used to modify the headers (and save into a new pcap) but it doesnt seem to support modifying payload.
Also modifying payload can be tricky since many protocols uses various methods of "encryption" (or rather encoding) - but sure to replace a string with another string "should" be easy (as long as you know the limitations of this) - however I didnt found any good rewriter of the payload stuff yet.
Edit: Some good examples regarding tcprewrite: http://chrissanders.org/2010/12/sanitizing-pcap-files-for-public-distrubution/
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