Hi there,
It's great you're creating custom templates for your estimates! Don't worry, I'll help figure out why you're unable to switch to a different template.
Based on what you've described, you're following the proper steps for switching estimates. One suggestion I have is making sure you're saving the estimate template after switching it. Once you've saved it, you can print/preview to see if it's working. If you're still having issues, I recommend clearing cache and cookies on your browser.Doing this will help fix most issues the site may be having. Here's an article which shows you how to do that: How to clear Cache and Cookies.
Give this a try and if you're still having issues, I recommend contacting our support team using this link.