Hi there,

if you have no settlers but construct a building; the building will show the missing settler exclamation mark; when you upgrade the building it will replace the missing settler exclamation mark with the upgrade building exclamation mark.

If the upgrade finishes before the next settler is due to arrive the building will now operate as normal even though in the building details it still shows the building as missing settlers but there is no longer the missing settler exclamation mark

You won't be able to immediately see it working until you one production cycle is complete and the amount in the mine decreases. (all done before the next settler is due to arrive)

Tested it out on a larger scale with 20+ wells and 6 copper mines, then upgraded the mines first while still checking the timer for the arrival of the next settler; also tested this on the well after the copper mines, this works for both.

Please try for yourself to verify.