Searching did not lead me to the answer of my specific question. So here goes...

Bought a non-refundable ticket (for illustration, let's say it cost $1000). Had to cancel. (Did it days before the scheduled departure; so I have a $1000 travel credit, no problem).

Am now ready to apply the credit resulting from my canceling the flight, and the price of the new set of flights including the $150 change fee is significantly less than the $1000 credit (again, for illustration, let's say the new flights cost $300 plus the $150 change fee, for a total of $450 ... leaving a residual credit to $550).

Do have to pay another fee to use that residual $550 credit on a subsequent flight?

What if my next purchase costs $300. Is there a (another) fee to use that remaining credit?

A call to the 1K desk told me I only pay the single $150 fee. No additional fees to use any residual credit. But I wanted to know if anyone has had contradictory experiences. Thanks.