Upgrade Azure App Service to from .net core 3.1 to .net 6.0

We have an azure app service... which was created a few years ago using .net core 3.1... and when you navigate into the portal the .net version is blank.

If I query via the cli eg

az webapp config show --resource-group myresourcegroup --name myazureappservice

I get "netFrameworkVersion": "v4.0".

So I can change the value to .NET 6 via the UI but if I need to rollback I'll need to do so via the cli using

az webapp config set --name myazureappservice --resource-group myresourcegroup --net-framework-version "v4.0"

But then the drop is no longer blank but appears as:

So is that actually rolled back?? Pretty sure blank (.net core 3.1) != .net 4.8?!

As mentioned in MSDOC, .NET Core 3.1 version is out of support. That could be the reason you are seeing these changes in your web app.

I set the version of my web app to 3.1 to check the same:

.NET version was blank as you highlighted above:

But the other versions were available in the dropdown, we can opt the required version:

>So is that actually rolled back??

Yes, the version is getting reflected in the .NET version of Stack Settings. So, the roll back happened.

>Pretty sure blank (.NET Core 3.1) != .NET 4.8?!

Yes, you are right. .NET Core 3.1 and .NET 4.8 versions are different.