Converting database tables using threading

Firstly, using threads doesn't quicken anything up, unless you have multiple processors or cores.

If anything, it makes things slightly slower because of the cost of managing threads - in particular 'context switches' between threads.

The only good reason to use them in a GUI application is to prevent the GUI from freezing whilst a long running or processor intensive operation is running in the background.

If that's what you're really worried about, then I'd check out the BackgroundWorker class:

http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.aspx