As you probably know, the "global" memory space is not limited to the gobal field values. Once a file is hosted, there are number of things that can't be modified permanently anymore (unless, of course, you download the file, modify the setting you want and re-upload).
Here is a checklist to save you time and frustration.
This is great content @FabriceN . It looks as if we like the same topics. The link below is a wiki, should anyone want to edit accordingly:
I saw an interesting clip on youtube a while back about a way to store global values within a hosted solution. I can't find the video, but the technique is basically make the global field into a global calculation field instead that is set to the value you want to store. Then change it back to a global and it should retain the value of the calculation and persist when the file is closed. A very cool technique for saving time taking the file offline.
yes, there are a couple of ways to change a default value of a global.
I think the one you mention is the best one but it shouldn't be made a global calc but a stored calc (I'm not 100% sure but it seems logical)