UDesign Theme Update Question

Hi, I have just volunteered to look after the website of a charity (I am not a developer so having to learn fast!!).
The website uses the UDesign Theme but is on version 2.5.4 and I want to update it to the latest version.
The people that developed the site are no longer around and I dont have any any details of the Envato account that was used to purchase the original UDesign license.
I have registered a new Envato account, but my question is this - if I now purchase a new UDesign license and download the code, will I be able to update the theme on the website or will it still somehow need to be associated with the account used for the original purchase?
Also are there likely to be any compatibility issues with going from an old version to the latest one?
Any help and thoughts gratefully received.

Given the copy you are using will be around 4 years old, jumping straight from there to the latest copy is very likely to cause problems.

The right way to do it is to create a staging environment, back up the site etc and then at least you can revert back to where you are if it falls apart.