Portability & blend-and-extend
If you're moving, porting lets you carry your existing mortgage and its rate to the new home instead of breaking the contract and paying a penalty, and blend-and-extend lets you fold in extra borrowing at today's rate without that penalty either.
When you sell one home and buy another mid-term, you have two choices. You can break the mortgage, which usually triggers a prepayment penalty (the lender's charge for ending the contract early, often thousands of dollars on a fixed rate). Or you can port it: keep the same rate, balance, and remaining term, and move them onto the new property. Porting is not automatic. You stay with the same lender, you re-qualify from scratch (including the federal stress test that checks you could handle a higher rate), and you close the sale and purchase inside the lender's porting window, commonly 30 to 120 days. Expect a small admin fee, roughly $100 to $300, which is far cheaper than a penalty.
Most people moving up need a bigger loan, and that's where blend-and-extend comes in (also called port-and-increase). Your old balance keeps its old rate; the new money you borrow is priced at today's rate; the two are combined into one blended rate that sits somewhere between them. Say you owe $400,000 at 2% with three years left and need $500,000 for the new place. The extra $100,000 is priced at the current rate and blended across the whole balance. Many lenders also reset your term to a fresh five years as part of the deal, which quietly extends how long you're locked in. The word "extend" is doing real work there, so ask whether your term is being reset before you agree.
Blend-and-extend isn't only for moving. If rates have dropped and you want out of a high rate without paying a penalty, a straight blend on your current home mixes your old rate with today's lower one, no penalty charged. Moving to a cheaper home works too: a port-and-decrease shrinks the loan, and any shortfall can sometimes be covered by your annual prepayment privileges instead of a penalty.
The catch is that not every mortgage can do this. The deeply discounted "no-frills" or restricted products some banks push almost always strip out the portability clause, and some lenders won't port a variable rate at all, or make you convert it to fixed first. That's the part worth naming plainly: a rate that looks a fraction of a point cheaper today can cost you thousands later if it can't move with you. The portability terms are written into your contract from day one, so they're worth reading before you sign, not after you've listed the house.