I would push them to go with 10-digit dialing, it's coming faster than you think. Yeah we've been hearing it for years, but there are a lot of areas already there and quite a few coming up on the slate in the next few years.
Here is a spreadsheet that gives you some idea of what is coming in the next 1 - 3 years. Keep in mind this thing is updated at least quarterly if not monthly.
The way it is right now, all cell phone users regardless of location should already be putting all numbers into their phones as 10-digits. This not only applies to text messaging, but to assure you can place calls to people in your address book regardless of where you are.
The reality is that we might as well get used to it because it is coming, and we are already way behind Europe in the implementation of it.
My suggestion would be to do exactly what the phone companies are doing. Set up the dialing plan on the switch so that it supports both 7 and 10-digit dialing and give them a cutoff date maybe 6 - 9 months into the future. Build a company-wide campaign to educate the staff on the new procedures - make games and contests out of it or catchy jingles, etc... For example, in the late 90's when our area code 414 was split into 414, 262 and 920, AT&T used the jingle "920 so you can grow" to promote the change. We used it to in a lot of our internal communications, etc. Encourage people to program speed dials for frequently called numbers.
On the date you have set aside for the switch just remove the 7-digit dial strings from the dialing plan on the system and you will be set to go.
This really won't have anything to do with your internal dialing plan. If you are using 4-digit extensions nationwide the phone company will still send you 4-digits after the change, so there is nothing to worry about there.
I just sent out a memo to our organization last week telling them it's going to be mandatory in my part of WI in 2012, and northern WI will be mandatory in 2010 already. I suggested our marketing folks be sure to always publish the full number, and to check their faxes and other dialers.
Everything you need to know about the progress of the sweep across the nantion is available on that NANPA website if you look in the right place.