I had one layout with a mixture of "O" and "027" track. I just squeezed down the opening in the "O" track to accept the smaller pins.
I did this by putting 027" pins in the opening, and squeezing the web underneath the rounded part with a pair of needle nose pliers. The pliers had a slight taper, so I worked them in further and further, until the pin was held tight.
The transition was smooth.