From what PHP version to what PHP version you are going to upgrade?
You shouldn't have any problem with updating the PHP version. The minimum PHP requirement for VM2.6 is PHP5.3.10
As long as you have PHP5.3.x (higher than PHP5.3.10) you will be good to go and you don't need any upgrade.
You may receive some Strict Standards error (mainly caused by Joomla Core Files) if you jump to extremely new PHP versions (such as PHP5.5.x)