MySQL: Enabling innodb_undo_tablespaces
Probably refers to "fresh installation".

The workaround is painful...


Dump all the data.
Stop mysqld.
Blow away ibdata1 and iblogs.
Change the setting in my.cnf.
Restart.  (This should feel like a 'fresh' install.)
Reload the data.


Do you have some indication that changing that setting will help you significantly?  If so, please explain.  It is a very new feature, and is there for a reason, but not many users need it.

Also

from the 5.7 documentation, - dev.mysql.com/doc/refman/5.7/en/innodb-undo-tablespaces.html  


  The innodb_undo_tablespaces configuration option is deprecated and will be removed in a future release." and "The number of undo tablespaces can only be configured when initializing a MySQL instance and is fixed for the life of the instance."


– Thanks to @OutfastSource