Modify linked server definition (Oracle) in SQL Server 2012
How you solved the issue is the way to solve it because we can't modify a Linked Server. And SQL Server doesnt read the tnsnames.ora. Though the solution is drop and recreate we can use SQL Management Studio to edit the Drop and Create script as follows:
1. Right click on the linked server.
2. Choose Script Linked Server as.
3. Choose New Query Editor Window.
4. Now we can modify the script as our requirements and run. This simply drops and recreates the Linked Server.

Thanks