How to save traffic by changing page addresses?

Question

Search Engine Optimization

Given

Website in HTML
No directory and file structure

Required

Create a normal file and folder structure

Problem

When you change the name of the files, the address of the link to the page will change accordingly.
I'm afraid that everything will go to hell from the index, the site is visited, the traffic is high

Question

How to avoid leaving the index
How to replace page addresses in the index

Answer 1

Use 301 redirects.
A drawdown is acceptable, but, as a rule, within a couple of weeks - a month, positions are restored. Or maybe there won't be a drop at all.

Answer 2

Well, in general, this is problematic and is usually solved by a 301 redirect. But when I had to do this, I made a new engine and saved all the URLs from the old one in it