Migrating from a URL key to an MD5E key (after addurl --fast using an external special remote) fails:

(master_env_py27_v28) [03:15 PM /data/ilya-work]$ git annex --verbose --debug migrate A4.taxfilt.bam
[2018-10-29 15:15:55.197015] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","ls-files","--cached","-z","--","A4.taxfilt.bam"]
[2018-10-29 15:15:55.232756] chat: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","check-attr","-z","--stdin","annex.backend","annex.numcopies","annex.largefiles","--"]
[2018-10-29 15:15:55.232934] read: git ["--version"]
[2018-10-29 15:15:55.233841] process done ExitSuccess
migrate A4.taxfilt.bam failed
[2018-10-29 15:15:55.263931] process done ExitSuccess
git-annex: migrate: 1 failed
(master_env_py27_v28) [03:15 PM /data/ilya-work]$ git annex info A4.taxfilt.bam
file: A4.taxfilt.bam
size: 12.75 megabytes
key: URL-s12750247--dx://file-FJZjKx800YQPJ5j589Q2Kfzv/A4.taxfilt.bam
present: true
(master_env_py27_v28) [03:16 PM /data/ilya-work]$ git annex whereis A4.taxfilt.bam
whereis A4.taxfilt.bam (2 copies)
        0928dfcc-4dbe-4c24-a5a0-ac05c4a2c829 -- ilyas_main_repo [here]
        6803648c-87e6-41ef-9570-48adb13ff59a -- [mydx]

  mydx: dx://file-FJZjKx800YQPJ5j589Q2Kfzv/A4.taxfilt.bam
ok
(master_env_py27_v28) [03:16 PM /data/ilya-work]$ git annex --verbose --debug migrate A4.taxfilt.bam --backend MD5E
[2018-10-29 15:17:15.621264] read: git ["--git-dir=.git","--work-tree=.","--literal-pathspecs","ls-files","--cached","-z","--","A4.taxfilt.bam"]
migrate A4.taxfilt.bam failed
git-annex: migrate: 1 failed

fixed --Joey