Does Extension Change Note from Variable to Fixed?

We offer a variable rate loan product for a commercial operating line of credit. The variable rate language in the promissory note states that the rate can change on any day. We have a policy in place to allow the customer to extend the note prior to maturity, if more time is needed for repayment, usually thirty or sixty days. With the extension, we increase the interest rate one percent from the rate they were at at maturity. The extension form we use is provided to us by a major banking product suppier, and we simply fill in the blanks and have the customer sign the extension. Part of the form requires us to fill in an interest rate with a space to date when the rate begins and another space for the length of the interest period. We have always placed the new interest rate at one percentage higher in the interest rate field, and placed the date of signing as the start date with words we insert "until paid in full" in the space where length of term for the rate is. At the bottom of the form it states: "all other terms of the original obligation remain in effect". We now have a discussion going on as to whether the way we are filling out the extension converts the note from variable rate to fixed. We have always considered the extension as variable in the past, because of clause that states the original obligation remains in effect. Now, we are being questioned as to using the words until "paid in full" make the note fixed rate. Would you have any advise on this matter to set us straight?