The easy way to massively change the puppet master and puppet ca entries for a huge number of hosts is to use the API with a little script.
Here are 2 samples based on python and shell.

Sample script in python : https://gist.github.com/ekohl/5813331

Sample script in shell. You need to identify the good puppet master and ca proxy id, and replace the variables before using this script

#! /bin/sh
#

USER="admin" 
PASS="xxxxx" 
FOREMAN_URL="https://10.1.1.1/api" 
puppet_ca_proxy_id=3
puppet_proxy_id=3

curl    -s -H 'Accept:application/json' \
        -k -u "${USER}:${PASS}" \
        -G \
        $FOREMAN_URL/hosts | \
        prettify_json.rb | \
        sed -ne  '/"id":/s/.*: \([0-9]\+\),/\1/p'  | \
while read a ; do \
        curl -s -H 'Accept:application/json' \
        -k -u "${USER}:${PASS}" \
        -X PUT \
        -d "host[puppet_ca_proxy_id]=${puppet_ca_proxy_id}" \
        -d "host[puppet_proxy_id]=${puppet_proxy_id}" \
        ${FOREMAN_URL}/hosts/$a
done

Hi,

thanks a lot for the script. :)

Meanwhile I had to do it manually. :(

Best Regards,
Oliver