In the Advanced Rename pane, how to change the extension of files?

Clicking the checkbox for Ignore extension doesn't do anything.
I just want to change the extension of multiple files...

Thx

If the old extensions are all the same, this is probably the easiest way:

If the old extensions are all different, you can do it with a regex quite easily:

thx - I see
I went ahead and made a button to rename my extensions and bypass this window

but just for the record, the extension can't be changed in the preview pane at the bottom like the other fancy manipulations that one can do with the first part of the name?