Hi Larry, I'm with you!
In a market sense, you would deduct around 10K for the loss of a bedroom.
However, it is important that your home be as functional as possible while you are there (Especially if you are 5 years + away from needing to sell).
If you do a nice job with the conversion, I would almost expect a wash in terms of value. Also , when your home is being compared to other homes in the neighborhood, yours will stand out as being more functional and updated.
All the best with your project,