>I know of no explicit tool
>
>but

>USE your_frx
>
>replace CLASS with ctargett FOR CLASS=='OldCLASS' and BASECLASS='TEXTBOX' 
>use
>compile classlib your_frx
>

>is fast enough

You could do that, but you cant see the context and if you make a mistake you don't have an easy way to compare with the previous state what could be wrong and then you must start again from scratch using a copy, so this is one of the benefits of doing this replace in plain text with any editor, it is safer and you can visually see what are you changing in the correct context as PRG-like code, easy enough to understand for any programmer.