When i change the resolution of my terrain, all my prefabs get moved

First off im new to unity.

I have been making a City in unity 5 and the res is 1000 by 1000 but its not big enough. So I changed my res to 1250 by 1250 and all my models or prefabs got moved off the terrain where they were placed. Is there anyway to keep the models connected to the terrain and make the terrain bigger.

-Thanks