I have a rather large table (19,000,000 million rows = 7 gig). I need to alter the next extent size, as it is too large (.5 gig). I do not have that much contiguous free space in this file system. So, I plan:

alter table GLTRANS
storage (next 250M);

Does that sound okay?
Does the database have to be offline or can the users still be accessing it?
Does this come back quick, or is it going to try to dump and reload all the records?

Thanks in advance,