Houska nailed the answer to that question.
There are only two types of loans - a term loan and a demand loan. In a term loan, the lender/borrower can only change the loan conditions at the end of the term.
In a demand loan, the lender can change the loan conditions at any time by "calling" the loan -- ie, demanding repayment of the loan, in full, immediately -- and then setting up a new loan with that lender or another lender. That's why HELOC debtors had a practical choice of either acceding to the higher interest rate or paying the loan in full (which would mean getting a loan from someone else to pay off the demanded balance) -- everybody knew that the loan could be called at any time.
In our case, we "negotiated" the rate increase down from the proposed rate of Prime+1 to Prime+0.5, but we knew our bargaining position was quite weak.
Lenders love HELOCS for this reason. They sell it to you as something that provides you with additional flexibility "oh, you can "lock up" a HELOC into a regular mortgage at any time", but the fact is that this kind of demand loan provides the lender with the exact same amount of flexibility.
Most (all?) variable rate mortgage lenders allow you to choose to go from variable to fixed rate at any point during the term. But the people I know who have done that have discovered that there is a 3-month delay written in to the fine print, so that the value of anticipating a rate hike is diminished. Especially since the Bank of Canada tells their bank customers and members of the public at the same time about what to expect in terms of the overnight lending rate they will be charging to the banks (can't remember what it's called - Bank of Canada prime?), which in turn is used to set each bank's prime rate.