Overview:

# sh -x plymouth-set-default-theme -R
+ set -e
+ '[' -z '' ']'
+ PLYMOUTH_LIBEXECDIR=/usr/lib
+ '[' -z '' ']'
+ PLYMOUTH_DATADIR=/usr/share
+ '[' -z '' ']'
+ PLYMOUTH_CONFDIR=/etc/plymouth/
+ '[' -z '' ']'
+ PLYMOUTH_POLICYDIR=/usr/share/plymouth/
+ '[' -z '' ']'
+ '[' -z '' ']'
++ plymouth --get-splash-plugin-path
+ PLYMOUTH_PLUGIN_PATH=/usr/lib/plymouth/
+ DO_RESET=0
+ DO_INITRD_REBUILD=0
+ DO_LIST=0
+ DO_HELP=0
+ THEME_NAME=
+ '[' 1 -gt 0 ']'
+ case "$1" in
+ DO_INITRD_REBUILD=1
+ shift
+ '[' 0 -gt 0 ']'
+ '[' 0 -eq 1 ']'
+ '[' 0 -ne 0 ']'
+ '[' 0 -eq 0 ']'
+ '[' 1 -eq 0 ']'
++ id -u
+ '[' 0 -ne 0 ']'
+ '[' 0 -ne 0 ']'
+ '[' '!' -e /usr/share/plymouth/themes//.plymouth ']'
+ echo '/usr/share/plymouth/themes//.plymouth does not exist'
/usr/share/plymouth/themes//.plymouth does not exist
+ exit 1

Steps to Reproduce: 

 plymouth-set-default-theme -R

Actual Results: 

 No new initrd is built.

Expected Results: 

 initrd should build with selected plymouth theme

Build Date & Hardware:
NAME=openSUSE
VERSION="13.2 (Harlequin)"
VERSION_ID="13.2"
PRETTY_NAME="openSUSE 13.2 (Harlequin) (i586)"
ID=opensuse
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:opensuse:opensuse:13.2"
BUG_REPORT_URL="https://bugs.opensuse.org"
HOME_URL="https://opensuse.org/"
ID_LIKE="suse"

Linux hostname 3.16.6-2-desktop #1 SMP PREEMPT Mon Oct 20 13:47:22 UTC 2014
(feb42ea) i686 i686 i386 GNU/Linux

I found a workaround by editing the script directly to call the correct
function when passing the -R parameter:

        -R|--rebuild-initrd)
                get_default_theme
                DO_INITRD_REBUILD=1
        ;;