I think myprogram needs to release 1.1.0 which supports the myprog alias. If the user invokes myprogram then it should present a notice/warning to the programmer that this name will be deprecated in the next major version release. Additionally, 1.1.0 should exist long enough for people to become aware of the impending change. I highly advise against releasing 2.0.0 the day after 1.1.0.
Upon release of myprog 2.0.0, myprogram should no longer work. The release of 2.0.0 could be nothing more than a name change. This will help to make the transition easier for developers since they have to worry about just a single compatibility-breaking change.
An alternative route is to fork myprogram into myprog and issue an abandonment notice like PHPExcel did; https://github.com/PHPOffice/PHPExcel
Regardless, I don't think versioning is going to be the big stumbling block but rather the name change itself. It sounds like a headache especially if people come across old tutorials for myprogram and are not aware of the name change.