I wasn't able to reproduce. Here are the steps I took - let me know how your steps are different.
1. Selected a series of Process 2010 images in Library grid
2. From the Photo menu, selected Develop Settings > Update to Current Process (2012)
3. After the update finished, deselected all images.
4. Clicked on one image in the grid, held the shift key, and clicked on a different image.
For me, all images in the range I clicked were selected.