No migration, no cry

There is no uSync Migrations extension for moving forms from 7 to modern. There isn't even a uSync Forms for V7. However the serialization format for forms on disk didn't change (much?) from 7 to 8, so we can do a trick. (Thanks to Kevin for the tip)
We can do 7 to 8, then 8 to modern.
This applies to files. I have not tried with the forms in database method, but that should be possible to do with database migrations from 7 to 8.

7 to 8

Create an empty V8 site and install the following nuget packages:

• UmbracoForms
• uSync.Forms

To "migrate" your forms from 7 to 8 you just copy the files over. 😁
Your source files should be in /App_Data/UmbracoForms/Data.
The target should be the same in a V8 site.

When your files are copied you're ready to go into the backoffice.
Go to Settings \ uSync and click Export.

You should now have uSync files for the forms under /uSync/v8/Forms.

8 to modern

Install uSync.Forms in your modern site.
Copy your uSync forms files from the V8 site into /uSync/v9/Forms.

The uSync files will have prevalues in a parsedPreValues field instead of preValues.
uSync Forms import in modern does not like this.
So I made a small powershell script that does the necessary "migration":

param(
    $path
)

$files = Get-ChildItem -Path $path -File

$files | % {
    $fileInfo = $_
    $doc = [xml](Get-Content $fileInfo.FullName)
    $pageNode = $doc.DocumentElement.SelectSingleNode("Pages")
    $origCData = $pageNode.FirstChild
    $pages = [array]($origCData.InnerText | ConvertFrom-Json)

    $results = $pages | % {
        $page = $_
        $page.fieldSets | % {
            $fieldSet = $_
            $fieldSet.containers | % {
                $container = $_
                $container.fields | % {
                    $field = $_
                    if ($field.parsedPrevalues.Length -gt 0) {
                        $field.preValues = $field.parsedPrevalues
                    }
                    $field.psobject.Properties.Remove("parsedPreValues")
                }
            }
        }
    }

    $cdata = $doc.CreateCDataSection((ConvertTo-Json -depth 99 -inputobject $pages))
    $replaced = $pageNode.ReplaceChild($cdata, $origCData)

    $doc.Save($fileInfo.FullName)
}

I called the script fix-usync-forms.ps1 and we can call it as such:

./fix-usync-forms.ps1 [path-to-site]/uSync/v9/Forms

That's it!

We can now go into the backoffice in our modern site.
Go to Settings \ uSync and click "Import" under Forms.

Success

Now all that's left is porting your potential code and views.

Happy migrating! 🙃