Is it possible to change the method used by a route from middleware in Laravel 5.3?

Let's say that I have an endpoint in Laravel 5.3 routes\api.php defined as below:

Route::group(['middleware' => ['api.preprocess']], function () {

    Route::post('/api/v1/notification', 'Api\v1\Controller@notificationPost');

    Route::get('/notify/v1/notification', 'Api\v1\ApiController@notificationGet');

});

Inside of, my api.preprocess middleware group, I'd like to change the method used by the request from GET to POSTsomething like this:

public function handle($request, Closure $next)
{
    if($request->some_var){
        Route::setMethod('POST');  // pseudo code, but you get the idea
    }
    return $next($request);
}

Let's avoid an X Y problem. I have a working API with the above get and post routes. I need to call the POST endpoint of the API from a 3rd party system that only sends webhooks as GET requests.

My first thought is to simply create a middleware to detect the GET request from this service and simply update it to POST and send it on it's merry way through rest of the routing.

Can I do that? If so, how?

NOTE: I know how to update request input, I want to change the actual method

via Chebli Mohamed