Is it possible that's not the original wheelset? I've run into a few 19" early 80's C-dale bikes and I don't remember them with anything other than 27" rims.

The '83 catalog states the ST500 would have come with Wolber Super Champion 58's in 27x1-1/4" with frame sizes 19, 21, 23, & 25. I don't see a footnote stating the 19" came in a different wheel size.

There's an off chance that a 700c wheel might work in the frame but you might not be able to adjust the brake pads low enough for the smaller rim. So a 700c could theoretically work with longer reach brakes and would certainly make a wider range of tire and tube choices available.