ASP.NET upgrade from .NET 4.0 to .NET 4.8

I'm upgrading a legacy ASP.NET application written in VB.NET and running on .NET 4.0 to .NET 4.8. I'm not familiar with the structure.

Note: I'm in an extremely controlled environment, and I'm not permitted to upgrade the project to a modern .NET SDK. That is, I'm only allowed to go to .NET Framework 4.8.

After I re-target to the project to .NET 4.8, how do I go about upgrading the dependencies?

Frow what I can tell, it looks like I'll need to update the assembly versions in each .aspx file:

<%@ Register Assembly="System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
    Namespace="System.Web.UI" TagPrefix="asp" %>

There is also a web.config file, but the assembly references seem redundant:

<compilation debug="true" strict="false" explicit="true" targetFramework="4.7.2">
  <buildProviders>
    <add extension=".rdlc" type="Microsoft.Reporting.RdlBuildProvider, Microsoft.ReportViewer.Common, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
  </buildProviders>
  <assemblies>
    <add assembly="Microsoft.ReportViewer.ProcessingObjectModel, Version=8.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
    <add assembly="Microsoft.ReportViewer.Common, Version=8.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
    <add assembly="Microsoft.ReportViewer.WebForms, Version=8.0.0.0, Culture=neutral, PublicKeyToken=B03F5F7F11D50A3A" />
    ...
  </assemblies>
</compilation>

Note: the web.config file for production targets .NET 4.0.

I deleted these assembly references in the web.config and it made no difference, at least to the build and start-up. Touching the assembly reference in an .aspx file (e.g. setting a bogus assembly version) did make a difference, and caused the build to fail.

Am I missing something? Are those assembly references in web.config required? Do I really need to update every assembly reference in every .aspx file manually?

Aside, if I try to re-add an assembly in the 'References' tab of the 'Property Pages' I get an error:

The website is already referencing the assembly System.Web.Extensions

I don't see the assembly references in web.config updated. It does work for an assembly that isn't imported in the .aspx pages (like System.Web.Extensions.Design).

Here's a glimpse of the solution folder.

Updated the .net version from web properties. Try build the application. Then you gotta resolve each files / modules seperately which includes breaking changes. As per my knowledge there are not alternatives or shortcut.
I have actually worked on a application where we migrated a .net 3 application to .net 4.8 written in vb, webforms. We actually had to resolve everything piece by piece.