Bulk change contacts in O365. Change display name to Last, First instead of First Last

I have 4000+ contacts I've imported into a customers O365 environment using a csv file and PowerShell. The custome now wants them all listed with Lastname, Firstname as their display name. I can do it for a single user with the following command:
set-contact "John Watson" -displayname "Watson, John"

How can I accomplish this for all of the contacts at once?

something like?
set-contact firstname lastname -displaymane lastname "," firstname

Thanks for the help!