I was hired to repair MRI units; I have 20 years repairing Radiation Therapy equipment.
Due to my not having any prior experience, I was asked to lower my salary requirements.
Eager to start a new career I lowered my salary by $7,000. My first two years I did 99 % MRI work, the other Radiation Therapy. I was offered a position closer to my home, which I accepted. Six months later I find that now I was transferred from a MRI job to a Radiation Therapy job.

Is this a bait and switch?

What you're asking is whether TWO YEARS AGO your employer purposefully hired you to repair MRI equipment, full well knowing that two years down the road a position closer to your home would become available, that they'd offer it to you, that you'd accept, and that it would result in your being transferred to the radiation therapy group.

No, this is not bait and switch. I don't know why you were transferred to the radiation job - you'll have to ask your employer about that - but nothing illegal has transpired.