It's not about changing the due date (or sla etc.) quickly, I think it's the way how you change it. The way could be easier and that's the reason why. A more intuitive way (inline edit / dropdown list) would be great. And saving time when doing it quickly is a positive side effect (especially if you sum the time up for all people).
Some guys from our department for usability of software etc. (yes we have an own department for that ^^) which we asked to take a look and provide us some feedback regarding the usability of osTicket also told us that it could be easier / more intuitive. Btw. that reminds me I still have not forwarded that feedback (or the interesting parts of that Feedback) to Jared and Peter to give them some view of guys who do usability optimization all day long with websites, "normal" software, apps (ios, android, etc.) and so on.