[AccessD] Change Field Type

Dear List:
 
I am trying to change the type of a field 'StartBase' in table
'MatterActions' from Number to Boolean.  I've tried two approaches:
 
1) dbs.Execute "ALTER TABLE [MatterActions] ALTER Column StartBase Boolean"
givers the error 3381 There is no field named 'Boolean' in table
'MatterActions'

2) Set tdf = dbs.TableDefs("MatterActions")
tdf.Fields("StartBase").Type = dbBoolean
 
gives the error invalid operation on the second line.
 
Does anyone know the correct code to change this field's type?

MTIA
 
Rocky