How to migrate content of one content type to another content type I have two content types. Content type A and Content type B

I have some nodes of content type A and want to migrate them to content type B but the content of type A should remains in the system.

I have tried the node_convert module but it simply changes the node type of the existing content as the node ids remains the same.

Is there any other module or some hint to move into a custom solution?
You can use Feeds module or Migrate module. First you export the Type A into a CSV for example, then build an importer that imports data into the Type B and use the exported CSV file. 

Be careful you choose the correct bundle for the importer (Type B) and to create new nodes. Check that your Unique field really is unique.