Feel like that is more of a question you should directly ask the admissions. TBH, 20 extra hours does not seem significant. I noticed most DO schools do not care about how many hours you have shadowed but more of what you got out of your experiences while shadowing + letter.
Unless there is something significant in the 20 hours that you did not know before applying to medical school or the letter will shine light on something new that is not within your application I don't think it will make a big deal, it might even delay your application a little. But it is definitely a question to ask admissions!
As long as you shadowed a DO and know the minor differences between MD/DO and large similarities between them and why those minor differences led you to DO prior to applying, you will be fine.
Good luck!
Edit: I am nice try. I just reread it. For some reason I thought you had additional DO hours. But you wrote you had MD hours. Then yes, I would call and try to update your application. But also try to explain why you applied to DO schools without shadowing a DO and what made you shadow one later on. They might ask that.