The bond you have with your first year housemates can be intense, but try not to get carried away with the excitement of moving to university and meeting new people. No decisions made at 3 a.m. on the walk back from the student union are wise decisions. Resisting the urge of sorting your second year accommodation and tying yourself to one group of people too early on might feel irresponsible or mean, but it's really not.
You're going to meet plenty of new people throughout the year, and you may end up not enjoying the experience of living with this group of friends. Take it easy, and don't panic about sorting accommodation until after Christmas. I fell out with my housemates in the final term of first year and was left scrambling to find new accommodation in the weeks before the summer break began. I managed, but it would've been easier if I hadn't made the second year pact so early on.