Issue using _doc as type name in dest index when moving data from multi type index in 5.6.10 to single type index in ES 6.3.1

[Gautam_Priya]

TL;DR: This question is about the problem -

"cause": {
        "type": "illegal_argument_exception",
        "reason": "Rejecting mapping update to [account] as the final mapping would have more than 1 type: [Account, _doc]"
      }

I have a multi type index in 5.6.10 that I am attempting to migrate to 6.3.1 by creating 7 single type indices instead of the single index with 7 types - thinking that better bite this bullet now than wait till ES 7.x to do this. Now to that end, I have successfully created the indices in the 6.3.1 cluster. However, when I attempted to migrate data from the multi-index + type to the single index hosting that type in 6.3.1 I ran into the error at the top of this post. Here is an example:

POST _reindex
{
  "source": {
    "remote": {
      "host": "http://localhost:9200"
    },
    "index": "multi-type-index",
    "type": "Account"
  },
  "dest": {
    "index": "account"
  }
}

The multi-type-index contains an Account type and the account index contains a _doc type identical in structure to the Account type. Attempting the _reindex operation above, results in the error below.

"cause": {
        "type": "illegal_argument_exception",
        "reason": "Rejecting mapping update to [account] as the final mapping would have more than 1 type: [Account, _doc]"
      }

Note that when I change the type name from _doc to Account in the single type account index, the _reindex operation succeeds.

Now I am trying to keep the _doc type name in the account index per elasticsearch guidelines. How can I achieve this without having to jump through hoops.

[Gautam_Priya]

Pointers on ways around this problem would be much appreciated.