Go to the beginning of the buffer an call query-replace-regexp. You can call query-replace-regexp by the menu item "Edit → Replace → Replace Regexp". Alternatively you can use the key-sequence M-C-% that is shown beside the menu item.
Use ^[^[:space:]]+ as regular expression (regexp) and \&@ as replacement string.
The first caret in ...
You can use org-number-to-letter from org-table.el (requires at least Emacs 26.1 or Org 9.1.9?)
;; => "Z"
;; => "AZ"
And type C-M-% (query-replace-regexp) with
[0-9]+ → \,(org-number-to-letters \#&)
If you need to do the search-and-replace from Lisp, use re-search-...
104 would be cz I think.
Using org mode eval setup:-- I suspect there are better choices fr functions but I'm an elisp learner.
These conversions might be more appropriates in your code in that there's less if/thens based on ranges.
(setq c 104)
(setq lsc (char-to-string (+ (% (- c 1) 26) (string-to-char "a"))))
(setq msc (...