different axle widths

Hello Group,

My bike came with rear wheel A. I fit rear wheel B to it -- no problem.
Now I tried to put A back on, and it did not fit.

I suspect B's axle is wider than A's. When B was mounted, the (steel)
frame was forced to become wider. The frame could not deform to be
narrower to fit A. I did not measure the axles; it was my guess.

What is the remedy?

Thanks.

Miles