I have an old Freehand file and it has a rectangle with the Pantone 187 CVC. I use 2 different ways to update with multiple versions of Illustrator:
I export the Freehand file as an .eps file and open it with Illustrator CS6. I select the rectangle, I go to the swatches panel and effectively see it is 187 CVC. I double click the icon of the Pantone color and it gives me spot color C:0; M:90,98; Y:72,16; K:23,53.
I export the Freehand file as an Illustrator 7 file. I open the Illustrator 7 file with Illustrator CS6. I select the rectangle, I go to the swatches panel and it is a Pantone 187 CVC but it has a very different aspect compared to the other file from the original .eps. So I double click the icon in the swatches panel and it gives me spot color: C:27,11; M:100; Y:81,03; K:27,24 (If I go to an Illustrator CS4 or CS5 and I open the Freehand It gives me the same color situations).
How is it possible that the same Pantone in the same versions of Illustrator (CS6) gives different CMYK values and clear screen differences?
If you want accurate Pantone to process conversion, look at a Pantone Color Guide.
Each release of software generally results in new digital Pantone color books with different color breakouts. This in addition to varying monitor calibration and application color profiles can cause shifts in colors. Your exported .eps may have been tagged with a color profile and Illustrator read and converted that upon opening the EPS. The AI7 file probably was not tagged, and therefore AI created color breaks based upon it's default color settings.
The only truly accurate way to convert a Pantone Color to a Processes Color is to refer to a printed Pantone color guide.