If I were to get a 21 spline TC can it be converted over to a 23 spline?

Yes, you'll need a 23 spline input gear. This involves complete dismantle of the transfer to install.

You also need to ensure the input gear length and gear pitch on planetaries are the same. If your getting a 21 spline XJ transfer you should be ok, if it comes from a ZJ make sure you compare parts closely.

Not a bad job, and it will help you to familiarize yourself with the internals of your transfer.