What is the best way to roll out a new version of the site instead of the existing one?

Question

Web development

Good evening.
A couple of months I bought a site - a bulletin board. It is made very old-fashioned, almost not adaptive for mobile. I understood this before buying, but since I needed a domain more (good age, sounds nice, etc.), and the price is symbolic - I bought everything (hosted site + domain + access to payment system accounts).
Considering that the code there is not readable, and it is impossible to finish the project, I decided to make a new site: with a nice design, adaptability, and all sorts of amenities. The problem is that I feel sorry for losing the existing audience, and now I'm at a crossroads:
1. The easiest way is to simply delete the files of the old site on the hosting and upload new ones, but this will definitely scare people away. Yes, and resetting the balance of users, deleting their accounts and ads is also not good.
2. A loyal option is to transfer people and all their data. It would be correct, but technically very difficult.
* It is necessary to transfer all users to a new database (about 6 thousand), reset passwords for everyone (so that people themselves set new ones when they first log in. On the old site, md5, and on the new one - bcrypt. I can’t transfer it imperceptibly)
* I need to transfer ads and pictures , private messages, reviews, etc. In short, the game is not worth the candle.
3. Alternative way: create a site with the same domain, just change the domain zone (say from .com to .biz) Throw a new site there. And on the old one hang a banner in the header - that this site is closed in half a year. During this time, use up your balance, and go to a new site, because it is cooler, safer, more convenient, etc.
This option seems to me the most adequate, but I am also afraid to frighten people away with such migrations.
Give some advice, how would you make such a transition?
Thank you!

Answer 1

if your site occupies a leading position in search engines, and this makes money, then I would not advise making sudden movements, you can lose everything at once and the site and positions and money.