In opposite to the old v7 file format, the fileposition has no direkt meaning. It is recreated each time a dgn file is opened and it's cache is filled. This means that using filepositions for any other purpose outside the current session is a more than a dangerous thing.
To keep it in the context of your question. The fileposition of each element, having a higher fileposition than the one to be deleted, will be changed. Elements with a lower fileposition will 'normally' keep their fileposition. The change will occure with the next refill of the cache (afaik).
HTH Michael