AFAIK although you can upgrade each drive one by one waiting the array to rebuild in each replacement, in order to resize them to use all available space you *need* to schedule downtime.
When you replace each drive, the new one will be only using the same size that it’s predecessor was using. This means that after changing all the drives you’ll end with exactly same storage available.
The next step will be to resize the partition(s) in that array. This means downtime and for that difference (250GB to 2TB) it’ll not be very short.