In most, if not all, aircraft that are Mach capable, there is a button on the autopilot pan that sets aircraft speed to IAS or Mach speed. Currently aircraft in IF fly IAS below FL280 and Mach above FL280, the change is automatic. I suggest an option in the general settings of IF to have it on manual or auto, auto being the default. There is an empty space next to the autopilot speed, and I think that would be a good place for a button that displays the option if manual IAS/MACH is selected.
I’m no good at photoshop or google draw or anything like that so I can’t make an example of what it would look like.