With Credit unions, it might be difficult to transfer a mortgage as they act within specific geographical boundaries. If you have a more complex mortgage (with a line of credit or cashback, for instance) or need more money, the game changes. In most cases you’ll be better off looking for a new mortgage, that will save you much more.
An extra attention should be given to the new mortgage regulations that came into effect in 2018. Under those regulations, porting a mortgage as is, becomes sometimes impossible, as the new lender has to reapprove the mortgage under much restricting rules