In some cases there is no need to have the home leased for a full year before you will qualify to purchase a different home.
If your income is sufficient and your debt load is low enough you could qualify for a different home even if the home has only been rented for a month or two.
I am currently processing the financing for a couple who already have four rent homes and are purchasing a new home for their primary occupancy dwelling. The current home they live in will not be rented by the time we close on their purchase. At the same time, their income is adequate to qualify them for the purchase of the new home while counting the entire previous mortgage payment against them.
In most cases when a home is counted as rental and it is producing income, the maximum allowable amount of the income that can be counted toward your positive cash flow is 70% of the rental income.
I would be glad to help you run some numbers for your and discuss your specific situation. Just let me know if I can be of assistance.