If you want your notepad window to become Office Word, do you simply write "Word" on the canvas and it changes?
You can't change an existing control by changing it's baseclass property, this is not possible at all, not only because the baseclass property is readonly. Even if it would be writable, what is loaded into memory is a textbox, and changing it's baseclass from textbox to combobox would not change the object. This is almost as changing the label of something and expecting it changed. I dare to suggest you simply write a needed amount of money on your wallet or notes (why is it called notes anyway?). You see: Nothing works this way, not in reality or in a programming language.
If you want something else you use RemoveObject() to remove the unwanted control and AddObject() to add the wanted control. Before adding columns by adjusting columncount you can also specify a memberclass/memberclasslibrary to tell vfp what to use instead of the standard column with the standard textbox.
In short, deep dive into OOP, VFP9 is not FoxproDos. Forms are not screens.
Bye, Olaf.