Assuming that the name of the subform control is subForm, and the combo box
(cboWhatSubform) returns the name of the form to be displayed as the
subform.
Me!subForm.SourceObject = Me.cboWhatSubform
Note that it's usually a good idea to reset the LinkMasterFields and
LinkChildFields properties when you change the SourceObject, even if they're
the same as for other subforms.