It is not an issue as long as you are not selling the property or refinancing. Your mortgage will continue as long as you do not break any of the covenants with the bank.

If you have equity on the old property, it would not be an issue as the net money you will get out of the sale can be used as a down payment for the other. If there is no equity, and it breaks even, then you should have sources of down payment allowed by the bank to purchase the new home. If your current property value is lower than the amount you owe, there will be a problem when you sell as you still owe the bank money. This answer is if you are in Canada. In the US, it is a different story as there are legal and creative ways in doing it.