Write DGN with the same cells as the source DGN

I think you need the cell library….Limitations: If the cell library referenced in the source DGN is not available to the writer, FME will use the default cell or may not replicate the cell exactly. Always verify that the seed file and cell library are correctly referenced

Otherwise on the reader, expose format attributes cell-related attributes (e.g., igds_cell_name, igds_cell_size etc.)

If you need to style or modify cell elements, use the DGNStyler transformer to set symbology and other format attributes.

Thanks ​@j.botterill.
So not possible then (at least yet). To do this type of task I will rarely have the original cell library

I have made a new idea if anyone wants to vote for this
 

The term cell library refers to a component of a DGN library that contains cells — usually one per model. Recall that cells are small drawings — usually of frequently-used or complex symbols. 

Cell libraries used with previous versions of MicroStation are standalone files in a special format that is not directly supported by MicroStation. Pre-V8 cell libraries are automatically converted to DGN libraries when attached to a DGN file in MicroStation, but they retain their extension, typically ".cel."

https://docs.bentley.com/LiveContent/web/MicroStation%20Help-v19/en/GUID-165DBAC6-20A7-D9E8-EBD6-EF98D1C776E0.html

This might be the community feedback from 9 years back 

I’m aware of what cells are, I’ve been using Microstation for years! Though I much prefer blocks from Autocad. They are always causing me problems.

Yes, that is the post I was meaning. I was hoping there would be a better way to do it now. I’m not going to be able to rebuild a cell library for each file, and the other options listed aren’t suitable.

I was hoping someone would have a workaround now. Perhaps how to recreate one automatically being that they are part of the original drawing

Essentially I want to take a file, make a small change, and then rebuild it easily so it’s identical to the original. In this particular case I’m just reprojecting the horizontal coordinates between two datums (~1.5m)

Indeed, I’ve had to refuse several projects which were basically reading semi-complex dgns, modifying som stuff and then writing them back in the same way as the original.