Can a script change the OPC item path of a OPC tag?
No, SQLTags stored internally cannot be altered in scripting. You have to use the SQLTags browser in the designer. However, if you create a SQLTags provider that is stored in the database you can edit them on the fly. Take a look at this link http://www.inductiveautomation.com/support/usermanuals/ignition/database_driving_provider.htm?zoom_highlightsub=database%2Bdriving%2Bprovider.
There are pros and cons for each type of SQLTags provider. Internal is very efficient and fast by cannot be altered dynamically. The database provider has to query a database all of the time so it is not as fast but can altered dynamically.