Re-Index Elasticsearch, ignore fields not in mapping - elasticsearch

Question

Trying to test out re-index API in elasticsearch and running into issues where existing data contains fields not present in the new index's strict mapping. Is there a way to tell elasticsearch to simply ignore those fields and carry on?
Edit: To clarify, by ignore I meant not to include those fields during the re-index process.

Answer 1

If you have access to the index settings before running reindex you can just do:
PUT test/_mapping
{
"dynamic": "false"
}
then change it back to strict once reindexing is done.
UPDATE based on your comment
POST _reindex
{
"source": {
"index": "src"
},
"dest": {
"index": "dst"
},
"script": {
"lang": "painless",
"source": """
ctx['_source'].remove('email');
ctx['_source'].remove('username');
ctx['_source'].remove('name');
// removing from nested:
for(item in ctx['_source'].Groups){
item.remove('GroupName');
item.remove('IsActive');
}
"""
}
}

Answer 2

While reindexing you can include or exclude source fields according to your destination index mapping.
To exclude some specific fields while reindexing:
POST _reindex
{
"source": {
"index": "source-index",
"_source": {
"excludes": ["exclude_a", "exclude_b"]
}
},
"dest": {
"index": "dest-index"
}
}
To include any specific field while reindexing:
POST _reindex
{
"source": {
"index": "source-index",
"_source": ["include_a", "include_b"]
},
"dest": {
"index": "dest-index"
}
}