Adding new field to the schema.xml

[#1]

I like to add new field to the existing document which have 1 million records.

I doesn't want to lose the exiting documents.

sample exiting doucment:

id : 123

field1 : 'sample1'      ---------------- indexed

filed2 : 'sample 2'    -------------------- stored

Now I want to add new field to the schema.xml which is only stored attribute and its not need to be indexed.

<field name="field3" type="string" indexed="false" stored="true" required="false" multiValued="false"/>

I know like this can be achived with the solrctl reload option.

But my doubt is after adding new field. is it neccessary to reindex whole documents?

what is reindex?

My understanding is again I have to load data to solr.and I don't have backup of these data anywhere

please help me on this

Thanks in advance.