There are a number of ways to generate a script file. First steps first though. Figure out the steps.
First Step Back up all your blocks as they are now. Nothing worse than automatically screwing all your blocks up and then not being able to get back to what you had.
Next step redefine the Nested block. Open it delete the Attribute definition from the block. Keep in mind what that definition is. You might even consider wblocking the definition out to a temp block to be used in the script file in moment. Make sure to pick an insertion point that makes sense.
Close and save the nested block.
Since we have our attdefinition in block now we can simply insert that exploded into each of our other blocks at the correct insertion point. Hopefully they are all the same location 0,0,0.
Work out the command sequence to insert the block exploded.
INSERT MYBLOCK 0,0,0 ;;
That should be all you need for the base of the script file.
save that to a file with the .scr extension where you can find it.
open one of the blocks you wish to redefine
use the scrip[t command to run it. Did it work? If so then we can move on. If not rework the Line above until it works as expected.
Remeber it is only bringing in the ATTDEF nothing else.
You will aslo want to add the CIRCLE Block Redfine in as well.
INSERT CIRCLE= 0,0,0 ;;;
The Circle Block mus be on the path for this to work or you will have to provide the full path to the block and name after the =
close the drawing and don't save, we are testing.
open it again and run the new script with both pieces. Did it work? keep both commands on one line, It will make the script file easier to build.
Open a DOS Window Windows Key + R -> CMD[Enter]
Change Directory to the folder where your 1000+ drawings are.
run the following DOS Command
DIR *.DWG /s /b >Myscript.scr
This will generate a script file base to start that contains all of your dwg files with their path.
Now Hopefully you have a good Text editor that allows for column editing. I use Jedit, Atom or even Visual Studio. JEDIT is pretty easy to use.
Open the myscript.scr file in your editor then append in front of every line. (you can also cheat and use replace text (Ctl+H) if you want.
Your lines should all have "OPEN " in front of them.
Now at the end we want to paste the script segment that we used to insert the attdef and redfine the block at the end. Make sure their is a space to separate the file name and the commands. Then at the end of it all we need to do is qsave and close the drawing.
You then just run the script file and AutoCAD will run through all your drawings and make the changes. I recommend testing on a small batch first. Remember you made a copy for back up. Alwasy copy those when you need to tray again. Don't make the mistake of moving your backup.
Hopefully you are familiar with this somewhat. Give it a go and let me know if you need some more help.
There are script generators out there, but I just use this method as it only take a few quick steps, once you know how to do it.
Good luck,