Well, at least this was the "safe" way round. Modern B&W film is highly tolerant of a couple of stops over exposure. I would probably just develop as for ISO 400, or might pull the development by 10%.(posted 9417 days ago)If the shots are really important, the best advice is to put the film to one side for now. Shoot another roll of ISO 400 at 100, under similar conditions (especially, with similar contrast), and try it with your chosen developer for a chosen time. If you don't like the result, try again with a different development time. When you are happy, you can develop the original roll.
If you haven't done any processing for a while, I really think a trial development is a good idea, to make sure you haven't forgotten anything critical.
If you just develop as if you had shot at ISO 400, the negatives will be a rather dense. But the shadows will be beautiful! If you pull the development, the highlights will be less dense, but the contrast will decrease. You have to choose which "fault" you want.
Film and developer manufacturers publish tables for their films and tables, and you can get more specific advice from them. As a general, very rough rule, subtract 10-20% for each stop of over-exposure. So you could could develop for between 65% (80% * 80% = 64%) and 100% of the standard time.
I haven't tried that film/dev combination. There are many people who may have done, such as http://www.chris-iris.com/.