Hi Fabio,

You can change SelectCmd only before calling CursorFill method. Every subsequent CursorRefresh will use same select command (and you should agree that if you change selected fields in your select command this would lead to a big mess).
So what you can do in your case is to write a "compound" where clause

WHERE (?ThisForm.Checkbox.Value AND tagField IS NULL) OR (?!ThisForm.Checkbox.Value AND tagField IS NOT NULL)

This is just the idea, but not a tested code, so you should play arrownd it.

Regards