Hello,
If you not choose to overwrite all files at the beginning of the hotfix process, the modified files should be kept and the new ones from hotfix package would be copied with ".new" extension, so you could merge your customizations with the hotfix changes.
The upgrade to higher version should probably follow the same way, however the changes may be larger and more "enforced"... we will have to wait for the actual upgrade utility and instructions, but I wouldn't be worried about that.
To further comment your questions in your first post: I would probably recommend to
clone the default
Calendar form control (in
Site manager -> Development-> Form controls)
with code in
~\CMSFormControls\Basic\CalendarControl.ascx.cs
and rename it to e.g. "BirthdayCalendar" - you will get another file name after cloning, but next to this file.
Then, in the Page_Load method, you will allow the limit and set the maximum limit and current selected date. It may look like this:
timePicker.UseCalendarLimit = true;
timePicker.MaxDate = DateTime.Now.AddYears(-18);
timePicker.SelectedDateTime = DateTime.Now.AddYears(-18);
With this setup you can achieve a year dropdown that will allow to input birthdate only if you're over (or just turned) 18... for example.
Other conditions can be "simulated" with this approach as well.
Hope this will help.
Regards,
Zdenek