Changing template and future strategies

Greetings,
I have two sites. One uses Corporate 2 and the other YourShop. Both seem to have lost support, correct?

Eventually I plan to update both sites to newer templates. They are currently using Joomla 2.x

My questions:

What is the best way to move an existing site to a new template? I, of course, want to test it completely before I make it live. I also want to keep all of the content and functionality, I simply want to make sure it is using a current, updated template. I would be happy with my sites if the templates were being updated. The only reason I feel like I need to make the switch is to stay current. Is this accurate?

Is it just par for the course that you will stop updating templates at some point? Do I have to just assume that I will have to change my sites every 2-3 years? Ideally sites are updated that frequently anyway, I understand. But, I am a one-person company that stays very busy with my sites as they are. I would probably update every 4-5 years if I had my choice.

Is there anything I should keep in mind when working with my site to make updating easier now or in the future?

Thanks so much for your reply.

Our templates usually have similar module positions so you just need to install new template via extension manager and then set as a default. You need to change the position only for modules published on special module positions like 'cart' etc., because not every template have this one.

Thanks. What is the best way to test the new template (with the content from the live site) before making it live while keeping the current site functional?

Also, is there anything different if the current template is 2.5x and the new template is 3.x? Will they both be able to coexist during testing?

Thanks again for any assistance. I haven't gone through this process before. My current sites are my first exposure with Joomla.

1) The best way is duplicate site because you can't have one site with two diffrent templates at the same time.
2) No, J!2.5 and J!3.0 are different templates for different Joomla! version.