How does a portable mortgage work?

From what I understand a portable mortgage means if I move, and there is time left on the mortgage term, I can take the mortgage with me to my new home… After I pay off what is owing on the first home.

What I dont understand though is how much. Lets say I started out with a $180,000 mortgage and I pay down $30,000 before I want/need to move. Assuming I sell my home for $180,000 and put $150,000 on the old house… Do I now take that $180,000 mortgage and its terms with me for a new house or do I only have the remaining $150,000?

I'd like to eventually be in a situation where I can use the original $180,000 amount AND the profit/equity from the old house to get something better. How would a portable mortgage work?

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