DPI, or more accurately PPI (pixels per inch) is just information how to scale pixels to a physical length measure, ie. PPI = number of horizontal pixels / physical width of the end result (or vertical pixels per height of the image, respectively). Hence, if you increase the size of your image and reduce the amount of PPI with the same ratio at the same time, your image data will not lose any quality. As long as you don't resample your image, the quality will always stay the same, because no image data will be touched, just the metadata.
Of course, the perceived quality of the image from the same distance drops, if you print the same image with half the PPI (and therefore double the width and height). But larger prints are usually made to be seen from farther away, so when a 300 PPI image is perfecly good down to a distance of approximately 10 cm (about the minimum human eye can focus to), 100 PPI image is still perfectly good from the distance of 30 cm.
Resize the image in Photoshop with resampling disabled (ie. tick off the Resample in the Image Size window). When you change the size, the PPI value will be adjusted automatically. Or vice versa, set the PPI to a desired minimum value and see how large the image would be.