As Andy suggests, make sure you read the manual. This command is totally outside DB2 engine ...
AFAIK, it updates some config files and does not do anything with the engine itself ...
If you are moving a container path, in theory, you can do with the database deactivated .. But, to be on the safer side, I would stop the instance.
I have used db2relocatedb successfully when the database was active. Of course, that was on dev and wouldn't do the same thing on prod.