Elasticsearch 5.1, used for the initial migration, does not support applying analyzers to keyword fields. It also does not support limiting length in text fields. As such we couldn't properly convert out current keywords to the new keyword type.
This should be fixed in ES 5.2, https://github.com/elastic/elasticsearch/pull/21919. This is expected to be released sometime in febuary, so hopefully we can do this before shipping to production. I'm not sure it's a strict requirement though, we are probably ok with a non-limited string field as these are mostly used to contain titles which are length-limited in mediawiki.