Make TextFormat "permenant"

When adding textFields dynamically, as well as the TextFormat, it seems that the textFormat needs to be applied every time the text is changed. So if I have:

TxtFld.text = ‘This is some text’;
txtFld.setTextFormat(txtFormat);

That works. But, if i add the line:

txtFld.text = ‘This is the new Text’

Then the format is lost. How can I make it so that I can change the text of the textfield, and not lose the formatting I’ve already applied?

Thanks