Level Number to Level Name

What version (e.g. v10.x.y.z) of MicroStation are you using?

I'm thinking the best method to perform this sort of operation would be to record a macro or VB code that can be applied to your list of "1000s" of DGN's using Batch Process.

eg.

https://communities.bentley.com/products/microstation/w/microstation__wiki/20301/change-color-of-all-elements-to-white-on-multiple-files

https://communities.bentley.com/products/microstation/w/microstation__wiki/2935/using-the-batch-process-utility-in-microstation-v8-cs

The batch-converter allows you to convert V8 files to V8, changing levels and symbology in the process. You can let Microstation create a CSV file for you, in wich you define the changes.

Ivo's suggestion of remapping through the batch converter is the easiest way I know to do this. It will handle many files at once, and it will change the elements from one level to another, according to a CSV file you can create.

The only downside to this is that it actually creates new files. I generates a converted copy of your original file.

If that is not acceptable (and I could see why it might not be) then Andrew's suggestion of a VBA macro that could be applied to your original files and make the changes there. It's a little more work, but it retains your original files as the only ones.
(I hate having "extra copies" of anything lying around)

I agree mary  but I do the export to c: \temp then examine they are ok and then archive> zip old files into superseded folder just in case and then move the temp ones to replace the old so no extra files lying around.