This PowerShell script will change your Sitecore instance search provider from Lucene to Solr or vice versa.

ChangeSearchProvider.ps1

function Get-ConfigFileFilter([string]$providerName)
{
	return "^.*\." + $providerName + "\.(.+\.)?config.*$"
}

function Set-SCSearchProvider
{
    $rootPath = Read-Host "What is the path of your Sitecore instance's website folder?";
    $choice = Read-Host "(L)ucene or (S)olr?";


    $validInput = $true;
    #test that path is valid
    If (!(Test-Path -Path $rootPath))
    {
        Write-Host "The supplied path was invalid or inaccessible." -ForegroundColor Red;
        $validInput = $false;
    }
    #test that choice is valid
    ElseIf (($choice -ne "L") -and ($choice -ne "S"))
    {
        Write-Host "You must choose L or S." -ForegroundColor Red;
        $validInput = $false;
    }
    

    If ($validInput)
    {
        If (($choice -eq "L"))
        {
            Write-Host "Set to Lucene." -ForegroundColor Yellow;
            $selectedProvider = "Lucene";
            $deselectedProvider = "Solr";
        }
        ElseIf (($choice -eq "S"))
        {
            Write-Host "Set to Solr." -ForegroundColor Yellow;
            $selectedProvider = "Solr";
            $deselectedProvider = "Lucene";
        }

        #enumerate all config files to be enabled        
        $regexp = Get-ConfigFileFilter $selectedProvider
        $filesToEnable = Get-ChildItem -Recurse -File -Path $rootPath | Where-Object { $_.FullName -match $regexp }
        foreach ($file in $filesToEnable)
        {
            Write-Host $file.Name;
            if (($file.Extension -ne ".config"))
            {
                $newFileName = [io.path]::GetFileNameWithoutExtension($file.FullName);
                $newFile = Rename-Item -Path $file.FullName -NewName $newFileName -PassThru;
                Write-Host "-> " $newFile.Name -ForegroundColor Green;
            }
        }

        #enumerate all config files to be disabled
        $regexp = Get-ConfigFileFilter $deselectedProvider
        $filesToDisable = Get-ChildItem -Recurse -File -Path $rootPath | Where-Object { $_.FullName -match $regexp }
        foreach ($file in $filesToDisable)
        {
            Write-Host $file.Name;
            if ($file.Extension -eq ".config")
            {
                $newFileName = $file.Name + ".disabled";
                $newFile = Rename-Item -Path $file.FullName -NewName $newFileName -PassThru;
                Write-Host "-> " $newFile.Name -ForegroundColor Green;
            }
        }
    }
}

Set-SCSearchProvider