With the availability of the Enterprise Plan 5 which includes lot of premium services, including Dial In Conferencing, Microsoft proposes a tool to update your dial in conferencing number if you have enabled the Skype for Business PSTN Conferencing license
You can get the tool from http://www.microsoft.com/en-us/download/details.aspx?id=47729 (32 bits version)
Then deploy the package (very simple installation steps, just “next, next, next”)
Then start the tool and follow the wizard
Select the user account for which you want to update the dial in conferencing phone number
And update your existing meetings with the new phone number
If no meeting appear, then you have nothing to update