An entity can be modified directly by changing its entity list and posting the changes back to the database.
The entmod function modifies an entity by passing it a list in the same format as a list returned by entget but with some of the entity group code values (presumably) modified by the application. This function complements entget. The primary mechanism by which an AutoLISP application updates the database is by retrieving an entity with entget, modifying its entity list, and then passing the list back to the database with entmod.
(setq en (entnext)) ; Sets en to first entity name in the drawing. (setq ed (entget en)) ; Sets ed to the entity data for entity name en. (setq ed (subst (cons 8 "MYLAYER") (assoc 8 ed) ; Changes the layer group in ed. ed ; to layer MYLAYER. ) ) (entmod ed) ; Modifies entity en's layer in the drawing.
AutoCAD must recognize all objects (except layers) that the entity list refers to. The name of any text style, linetype, shape, or block that appears in an entity list must be defined in the current drawing before the entity list is passed to entmod. The one exception is that entmod accepts new layer names. If the entity list refers to a layer name that has not been defined in the current drawing, entmod creates a new layer. The attributes of the new layer are the standard default values used by the New option of the AutoCAD LAYER command.
The entmod function can modify subentities such as polyline vertices and block attributes. If you use entmod to modify an entity in a block definition, this affects all references to that block which exist in model space and paper space. Attributes, unless defined as constant, are not updated for each block reference that exists in a drawing. Also, entities in block definitions cannot be deleted by entdel.