I found the answer.
I tried a small test case on my data on Solr-4.2.1. As I told, previously the type of the "searchtag" was text_general. I took a fresh Solr instance. There I defined searchtag as a text_general type field. There were also some other fields, I defined. Then I indexed some data into those fields as well as in searchtag. As it was a simple and straight forward way, so everything worked fine. I queried with searchtag and other fields and got the proper result.
Then I modified the schema.xml. I changed the type of the searchtag from text_general to text_ngram. After modifying the schema, I restarted the Solr instance. Now, I was not actually bothering about the data indexed for searchtag. Because I messed up with the schema for that field. So, I executed the same queries on the other fields. I'm happy that I got the same Solr documents as previous. Though I played with the schema, but other indexes for the other fields were intact. That's what I wanted.
Hope, this test case will help you in your area, if you stuck in this type of problem like me.
Note : I have mentioned the schema definition about "searchtag" and the field types in the question area.