Recently, I downloaded Godot 4 with the intention of opening a project I had originally created in Godot 3. However, this turned out to be a big mistake. Upon opening the project, I discovered that many things had changed, including tilemaps and the absence of the Tween class. Additionally, I encountered numerous errors and even corrupted scenes that I am unable to open. Although I have an old backup of the project, it hardly seems worth it to revert to that version, as doing so would require me to spend another week working to reach the point where I currently stand.
Any ideas to fix this problem ? did this happen to you too guys ?

Yes, I had to re-learn the new code and fix up my hierarchy and project, if it's any consolation things are more straightforward to implement now, but you have to learn it, I went through the pain in beta early on.

No question about it. I have a 3.5 project that I have been developing over 3 years. Hundreds of nodes placed in specific locations placed in 72 scenes, 30,000+ lines of code in C#. Upon opening my project in 4...useless. All nodes are resized, in different locations, individually handcrafted styleboxes gone. Code is completely upside down with an unsurmountable amount of errors to fix.
As much as I would love to upgrade, it would take me months to port, including countless hours of research to figure out what has changed. I wish the conversion process was already in place and user friendly.

Depending on the size of your project you could be in for much more than 1-2 weeks to upgrade to 4

It's a huge mistake that you did not make a backup. It even warns you before conversion. But it is what it is.

It took me 1 month to convert my project, and another month to smooth things out. And there are still a few issues that are missing/different than 3.5

You will have to go through every scene and make sure it is redone. Lookup some of the upgrade guides. Tweens will be one of the biggest annoyances.

I haven't had any luck with the auto-updater to 4.0. Every project I've opened (even simple ones) have been horribly corrupted.

I made a snapshot page on my website so that the Godot Version 3.5.2 software and documents are easily accessible. Please make sure that your source code is working and checked in to your code repository before attemting to upgrade to Godot 4. Also keep in mind that Godot 4 does not work with HTML5 (run in a browser) yet. Here is the snapshot page: https://www.aurorarealms.net/godot3

I wasn't sure if I wanted to upgrade. I was having issues with Godot 3. Getting fed up, with gdscript 1, I felt like I needed to restructure and organize my project.

I dived into Godot 4 because I figured I would enjoy re-working my game project.

I enjoy tweens, I prefer the new API. The UI performance seems better but I've got no idea how much they have improved UI performance.

IMO at the rate development is going, it will take another 4-12 months before v4 reaches feature parity with v3

cybereality I never actually stress tested Godot's physics before until I clicked that link. I was running 200 boxes under a virtual machine with one core passed to it, and it stayed above 30fps. That's awesome.