Upgrade API should also migrate index settings/mappings

[clintongormley]

Up until now we have maintained the ability to understand old mappings, but left the old mappings in place. Instead, we should migrate the old mappings to new mappings where possible.

If it is impossible to migrate mappings, then we should return an exception explaining the problem and instruct the user to reindex their data instead.

[clintongormley]

Handled on case by case basis