If you change PHP version with already installed builder then you will need to manually update builder afterwards (see documentation) in case if PHP version changed range (from the list of PHP versions ranges specified above).
If you have builder CRON set up (see documentation) then make sure that it is run also with the executable of the correct PHP version after builder manual update.
Warning:
When updating PHP version from PHP 5.* to 7.1.* or higher and manually updating builder then you may start getting warnings about missing "mcrypt" PHP extension when opening builder. To fix this issue open builder configurational file "config.json" and add parameter: "useOpenSSL": true, anywhere in the middle.