Open the EPS image in Photoshop and perform a Save As, select the format as Photoshop EPS. The EPS Options dialog box will display. In this dialog box select the Preview as Tiff (8 bits/pixel) and click OK. This will enable you to see the preview of an EPS changed through the Modify API.

Background:
Raster and Vector EPS images can be saved with either a Macintosh 8-bit (PICT) color preview or with a TIFF (8 bits/pixel) color preview (EPS files can also be saved with a grayscale preview, even if the EPS itself is a color EPS).