Definition
Mortgage portability is a contractual option that allows you, when moving, to transfer your existing mortgage to the new property without paying the breakage penalty. You keep the interest rate and remaining term.
Possible variants: (a) plain portability — balance and amortization remain identical; (b) port-and-increase — if the new property costs more, you can borrow additional funds at a blended rate combining the old rate and the current rate on the additional portion; (c) port-and-decrease — if you buy cheaper, you can reduce the balance, sometimes with a partial penalty on the prepaid portion.
Conditions to check in the contract: time gap between sale of old and purchase of new (typically 30-90 days), mandatory financial requalification on the new property, eligible property type (principal residence only at many lenders). Portability is particularly valuable when current rates are noticeably higher than your contractual rate.