Make the change because you like local stuff. The science behind the "food miles" argument is not that good. Most energy is consumed in the production phase. Transportation as a whole represents only 11% of life-cycle greenhouse gas emissions, and final delivery from producer to retail contributes only 4%.
Source: recent article in Environmental Science and Technology by Christopher L. Weber and H. Scott Matthews of Carnegie-Mellon:
"We find that although food is transported long distances in general (1640 km delivery and 6760 km life-cycle supply chain on average) the GHG emissions associated with food are dominated by the production phase, contributing 83% of the average U.S. household’s 8.1 t CO2e/yr footprint for food consumption. Transportation as a whole represents only 11% of life-cycle GHG emissions, and final delivery from producer to retail contributes only 4%. Different food groups exhibit a large range in GHG-intensity; on average, red meat is around 150% more GHG-intensive than chicken or fish. Thus, we suggest that dietary shift can be a more effective means of lowering an average household’s food-related climate footprint than “buying local.” Shifting less than one day per week’s worth of calories from red meat and dairy products to chicken, fish, eggs, or a vegetable-based diet achieves more GHG reduction than buying all locally sourced food." http://pubs.acs.org/journals/esthag/index.html