Just one additional suggestion ... check plugins for compatibility with destination core version of Moodle.
Above can be scripted and checked via moosh.
addons.txt file list addons by shortname:
Using moosh to check those using a bash shell script:
echo 'Add-on listing: ';
for i in `cat ./addons.txt`
echo "Addon in que: $i";
moosh -n plugin-list |grep $i
Themes are plugins and probably should be the first thing to research about newest version of moodle core code.
Be a good 'boy scout' and be prepared for a theme snafu.
Add a line to config.php commented out but ready to use in the event theme not compatible with latest/greatest ...