Construct doesn't really update automatically, every version is available at it's own address. The default shortcut just sends you to the latest one. Construct (which loads from cache so you don't have to redownload it every time), notices that the version at the link is different than the one loaded in your cache and tells you to refresh. You can bookmark and use a specific version forever.
Also if you find that you're unable to open a project in a newer version, then there should be no way for you to save in that newer version. The version you saved in should always be able to open the project (regardless of the addons you used in that version).
If an addon you have doesn't work with the latest version of construct, just use the one that it did work in.