I did what you have said, but it still doesn't allow me to merge changes. As a result I asked for Mike's help. Thanks for suggestion!In most cases you should be able to merge them.
If the system doesn't permit it, make a copy of your current templates and save them to your PC, revert the outdated templates, then add your customisations back.
You can use a program like Notepad++ to compare two files.
I would highly recommend dealing with any outdated templates as there may be other issues which aren't immediately evident.
Going forward, it won't be possible to provide support until you do so.