${parameter/pattern/string} behaviour changed

[dc]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc -I/home/abuild/rpmbuild/BUILD/bash-4.3 
-L/home/abuild/rpmbuild/BUILD/bash-4.3/../readline-6.3
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-suse-linux-gnu' 
-DCONF_VENDOR='suse' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I. -I./include -I./lib   -fmessage-length=0 
-grecord-gcc-switches -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector 
-funwind-tables -fasynchronous-unwind-tables -g  -D_GNU_SOURCE -DRECYCLES_PIDS 
-Wall -g -Wuninitialized -Wextra -Wno-unprototyped-calls -Wno-switch-enum 
-Wno-unused-variable -Wno-unused-parameter -Wno-parentheses -ftree-loop-linear 
-pipe -DBNC382214=0 -DIMPORT_FUNCTIONS_DEF=0 -fprofile-use
uname output: Linux hetz6 4.4.74-18.20-default #1 SMP Fri Jun 30 19:01:19 UTC 
2017 (b5079b8) x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-suse-linux-gnu

Bash Version: 4.3
Patch Level: 42
Release Status: release

Description:
        Command: x="a b c"; echo "'${x// /','}'"
        On old systems the output is:  'a','b','c'
        In this version the output is: 'a,b,c'
        This new behavior breaks some scripts.