Heating vent on suspended ceiling

The ceiling of my basement is now drywall. It's been intentionally broken up
for many times during the 40 years, like running cables and phone lines. I'm
going to change it dropped ceiling. But a headache is how to handle the
heating vents. The rectangle shape metal vents are right at the joists
(vertical position). The new suspended ceiling will be a few inches of
lower. My thought is to bring the vent lower by adding an extension, and
make a rectangle hole on ceiling tile, and place a cover. Hopefully, I can
find the same shape of metal duct as the extension in HomeDepot. Any advice