This method will have the least impact on existing numbers. I'm sure in the future, numbers will again run out, and you will be once again at this point. If you split and upset many people now and have to do it again in 5 years, I would not want to be the one to field those angry residents.
Business relies heavily on being able to be contacted.
Think of the lost busines because customers don't know their new number. 10 digit dialing is here. Within 5 years no matter what you do we will all be dialing 10 digits. 13 digits is next, my guess would be within 20 years. Plan ahead now.