How upgrading of a Odoo module works?
you can restart the server and start the server by python odoo-bin -d database_name -u module_name
or -u all to update all module
I have two databases in odoo DB1 and DB2. I made some changes to existing modules(say module1 and module2) in DB1 through GUI(web client). All those changes were stored to DB1 and were working correctly when I am logged in through DB1.
Now, I made some changes in few files(in same two modules module1 and module2). These modules need to be upgraded in order to load those changes. So, i logged in from DB2 and upgraded those modules. My changes in file loaded correctly and were working correctly when I am logged in through DB2.
But those file changes were loaded only for DB2 and not for DB1.
So, I wanted to know:

How upgrading of a module works?? Does it upgrades only for the database through which the user is logged in and upgraded the module?
And if it is so. Then, is there a way that I can Upgrade my module while retaining all the previous changes that i made through the GUI in that same module?
What are the things that are changed when a module is upgraded?