I'm using text that is set to "dynamic text".
In actionscript 3 I tried:
instancename.text = "abc";
var1 = "abc";
instance1._text = "def";
trace(var1 + instance1._text);
In actionscript 2, your lines should be
var1 = "abc"; instance1.text = "def"; trace(var1 + instance1.text);
(remove the _ char)
Moreover, try with
instance1.htmlText = "def";
this depends on how you have setup your textfield box.
Another important consideration: make sure that you have embedded the font in the textfield (Embed... button in Properties palette)