Hi Megan
One of the schools I have worked at did this just before I started. I think the process was:
* Keep the original four houses, and work out colours / crest / names of 2 x new Houses
* Incoming Year 7s were spread across the six Houses
* Current students were asked if anyone would volunteer to move across to the two new ones
* If nobody volunteered, I think some students were told to move, just to balance out numbers
* Have VERY enthusiastic staff members to lead the new Houses and increase House spirit
It did take a few years for the House spirit to be vibrant and start winning – but once those Year 7s were in Year 12, nobody knew any other way!