As you may know, when upgrading a DNN installation, it is necessary to merge the new release.config file with your current web.config file, because the new release.config may introduce/remove lines as important for proper functioning of the new DNN version.
The official merging procedure proposed by DNN authors is to cut your custom additions (connection strings, alternative data providers etc.) out of your current web.config and copy them into the new release.config, overwriting the corresponding generic/default parts. However, after some time, it may be a bit difficult to remember what modifications you have made to the original configuration file. Mostly it seems to be much easier to do it in the reverse way: copy new lines from the new release.config into your current web.config (and/or remove deleted lines).
To do the above, you need to know exactly how the default (i.e. unmodified) release.config of your current DNN version differs from the default release.config of the DNN version you are going to upgrade to. As I wasn't able to find this information anywhere on the Internet, I decided to put it on my website, because it seems to be generally useful for anyone upgrading a DNN installation.
With each upgrade of my site to a new DNN version, you will find there an article describing differences between release.config files of my previous and current DNN version.