I haven't seen your file but I assume that it is either TEXT or MTEXT you want to change?
Here's a really old one that use a user-defined expression to update any numbers within a string.
It'll show a dialog, where you write the expression and will then ask for objects. In this case, the expression would be (1- x) or (- x 1) in order to decrement by 1 (only one variable,
x, is valid).
"Really old" means that it never got finished (what else is new?) and it will blow up if you don't supply a valid lisp expression. It's only provided in case you're in a hurry to get the job done.