(entmod)ing an xref path

I am using the following code to try to soft path xref's automatically but
the "entmod" simply returns nil.

; returns a file name without a path
(defun get_file_name (file_name / string_length_orig string_length)
(setq string_length_orig (setq string_length (strlen file_name)))
(while (not (= (substr file_name string_length 1) "\\"))
(setq string_length (- string_length 1))
)
(substr file_name (+ string_length 1) string_length_orig)
)

(defun test ()
(setq first_loop t)
(while (setq current_block (tblnext "block" first_loop))
(setq first_loop nil)
(if (= (dxf 70 current_block) 36)
(progn
(setq xref_string (dxf 1 current_block))
(if (= (substr xref_string 2 2) ":\\")
(setq file_name (get_file_name xref_string))
)
;-----------------------------------------------------------------------------------------------------------------------------------
-----
;-----------------------------------------------------------------------------------------------------------------------------------
-----
(setq current_block (entget (tblobjname "block" (dxf 2 current_block))))
(setq current_block (subst (cons 1 file_name) (assoc 1 current_block) current_block))
(entmod current_block)
;-----------------------------------------------------------------------------------------------------------------------------------
-----
;-----------------------------------------------------------------------------------------------------------------------------------
-----
)
)
)
)


When done manually:
Command: !current_block
((-1 . <Entity name: 4050c938>) (0 . "BLOCK") (330 . <Entity name: 4050c930>)
(5 . "2597") (100 . "AcDbEntity") (67 . 0) (8 . "0") (100 . "AcDbBlockBegin")
(70 . 36) (10 0.0 0.0 0.0) (-2 . <Entity name: 4008b950>) (2 . "6602.00 bld01 -
XTitleB") (1 . "6602.00 bld01 - XTitleB.dwg"))

Command: (entmod current_block)
nil


Any ideas on why I can't entmod the path, is there some rule I am unaware of?
Cliff