With a little trickery all can be done in one operation Search/Replace
Search field:
{template}
Template:
{identifiers}
Search for:
(.*)(?:mobi-asin|amazon(?:_\w+)?):(B\w+)(.*)
Replace with:
\1asin:\2\3
Target field:
identifiers
Target identifier:
*
Also I advise you the
Mass Search/Replace plugin which will allow you to save and to redo quickly and easily this operation.