Did you just want to do this in form design, or in VBA;
You should just be able to open up the subform in design view, click on the text box (not the label for it) and change the Name Property.
Note that access tries to be helpful in cases where you are changing the capitalization fo a name only (if you try to change the name 'FIELDNAME' to read 'Fieldname', access will change it back to the way it was) , but if you change 'FIELDNAME' to 'xyz' then click away and back, then change 'xyz' to 'Fieldname', it will accept the change.
If you want to change the name of the field in VBA, you'll have trouble as you need to refer to the object by the name that you want to change.
Is this any help?
Mike