Elasticsearch version (bin/elasticsearch --version): 7.0.0 alpha1

Plugins installed: [analysis-phonetic, ingest-attachment]

JVM version (java -version): 1.8.0.181

OS version (uname -a if on a Unix-like system): Windows 2012 R2

Description of the problem including expected versus actual behavior: We have disabled "_all" field in our mappings in ElasticSearch 6.1.2. We have created indices and indexed data into it. The following is an example of the mapping.

{
"testindex": {
"mappings": {
"type": {
"_all": {
"enabled": false
},
"properties": {
"title": {
"type": "text"
},
"description": {
"type": "text"
}
}
}
}
}
}

Everything is fine with ElasticSearch 6.1.2. But when we upgrade to Elasticsearch 7.0.0 alpha 1, ElasticSearch boot up failed with the following error.

org.elasticsearch.index.mapper.MapperParsingException: Root mapping definition has unsupported parameters: [_all : {enabled=false}]

Is there a way to ignore it (_all) while booting up?
Or is there a way to update the mapping to remove the _all field?

Looking for a solution other than mandating re-index of data.

Please provide your suggestions. Thanks.