Editing Social Security Numbers from current numbers to some with XXXX inserted automatica

Hello all.
I am attempting to come up with a formula or something that will take social security numbers downloaded from another system in this format of 123-45-6789(Where I cannot fix the format) and automatically change them this format:
xxx-xx-6789

I need to do this for an entire column of numbers for 125 prospective customers.
Any help would be appreciated.

Sorry if this has been posted already, but could not find it.
Thanks.
Dave