If you are contemplating upgrading from the 7.6X to ubb7.7.4 then I suggest to change php to 5.6 .
Your current installed ubb version should still run and upgrading to ubb7.7.4 would be just plug and play.
Following the normal upgrade route..
Without playing games with switching back and forth your php version.
Then after the ubb upgrade you can switch to php7.X
