I also received the extension offer, although I am unlikely to benefit from it this year. If I don't extend it now, drop to gold and then likely drop to silver next year can I use the extension offer then (or the year after etc...).

Or is this a one shot at extension and if I don't use it now I'll lose it?

I was Diamond for many years pre-covid but since have not needed to travel for work. I have benefited from Diamond for personal trips (still have a ton of points) but not very many trips planned for this next year.