I would still prefer the extra work compared to the possibility of having upset customers who's websites are no longer working and when something is really broke it will take a lot of time to get fixed.
FWIW an example: I transferred a PHP app from MySQL 4 to MySQL 5 and found that the join clause changed it's behaviour extensively (less relaxed on the approved SQL syntax), breaking around 50% of the SQL statements in that app. It was a rather large app. It took me a week (admitted, not 100% full time) to update the application.
If you have customers with their own applications, being themselves responsible for these applications, I hope I'm allowed to give you the advise to do careful planning and communication. The hardware shouldn't be too much of an issue, but the upgraded versions of PHP, MySQL, maybe Apache2 vs Apache1 now, etc I would definitely test.