Is undoing Yosemite's CoreStorage conversion dangerous?
There are no obvious reasons to convert parts of the main drive(s) to a CoreStorage scheme - except for Fusion drives and FileVault 2 encryption - until now.

To convert your classical to a CoreStorage scheme boot to Internet Recovery Mode, Recovery Mode or an external bootable (thumb) drive, open Utilities/Terminal in the menubar and enter:

diskutil list to get the Disk Identifier of the partition to convert (usually it's disk0s2).

Then enter:

diskutil cs convert DiskIdentifier with the DiskIdentifier found in the step above.

The conversion will take some seconds ( SSD) or up to minutes (HDD).

After the conversion is finished, quit Terminal, open Disk Utility and check the result for errors. Then restart to your main volume and continue enjoying your Mac.