Not much any more. Mr.Patnams is correct. Basically, all the headers are open for data files and are recorded in Control files. They should be in sync. You need to down the database in second case, otherwise, log sequence and headers will be inconsistent, and control file can't read and match with it's current SCNs.
unless in the first case, you are making the required TS offline. so the headers of those particular DF are remianed same. after recovery, if the SCNs are matched with the headers, ur successful else u've second option, follow it. no other go...

hope u got it!!!!!