HS2 Ltd regularly review assumptions for the purpose of economic modelling.
The proportion of passengers predicted to switch from car and air to HS2, was last published by HS2 Ltd in 2013. This information is provided from page 82 of the document linked here:
The analysis will be updated to support future revisions to the Economic Case as part of planned updates to the HS2 Business Case. The proportion of HS2 demand predicted to have switched from air and car has not materially changed since the 2013 publication.