As shagemann said, there should be no problem opening your old template up in a new version.
I have been using the same template since version 8.5 and have just made sure that I have incorporated any new features that I need.
It really depends what you have in your template.There should be no need to update any attributes unless you feel the need to.
Your old libraries should work just fine - the biggest GDL change that was a bit of a problem was moving to 19 & 20. Since then it has been pretty good - of course there are new command you can make use of but probably no need to worry about them.
If you have a custom Work Environment, you will need to amend it to include the new commands & features, if not just use the new default WE.
Basically just open your old template in 24 and see what happens.
I am jumping from 22 to 24 at the moment and have not had to do anything to my template.
I have amended my WE, but that has nothing to do with the template anyway.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10