I think the focus needs to be on the roll out plan rather than statistics to justify a staff increase. If the number o f staff is a constraint, then plan the roll out with this constraint in mind.
Two of the key decisions in a roll out plan that I did not see raised in the original quesion are: Will we deploy to all users simultaneously? and Will we deploy all features on the new platform simultaneously? These are decisions that need to be made very early in the process as they will often affect the design and development of the product. Extra effort is usually required to have the old and new coexist, but this is almost always offset by using a phased roll out rather than a big bang roll out.
I do not think there are any industry standards onthe amount of support required for a roll out of a major product - there are far too many variables that will affect the number. If there is time, I would recommend staging the deployment by either users or functionality or both. If it is already too late, the best projection on the number of support calls will be the amount of support required by the test team (product difficulty) and the number of errors found during test (product quality).
