Some issues to be mindful about.
Drupal, has changed the way it works, and now, for path, it only requires you to have a SLASH.
Basically, what we call Path (/solr/index_name), in Opensolr, it should be / for your Drupal setup.
And what Drupal calls SolrCore, should be the Name of your Opensolr Index.
Also, if you use Opensolr in SolrCloud mode, please note that the solr server path is /solrcloud instead of /solr. (i.e.: https://server.opensolr.com/solrcloud/index_name/select?...)
So, unless Drupal has decided to hard-code the /solr part of the connection URL, you should be able to use your Opensolr SolrCloud Index, and set your path, in Drupal as /solrcloud, instead of / (slash).
Ultimately, we will help you upload your config files, regardless of the Solr Version you decide to use, as usual, we'll do this for free, and, instantly, during our office hours.
You might also want to watch the clip about all external integration issues.
It is a common misconception, that, if your Drupal module, or any other Solr plugin, requires, say, Solr Version 6.4.x, and Opensolr only provides Solr version 6.1.x, you can't use it with Opensolr.
With small modifications to your Solr config files, we can make your config files work on any of our Solr versions, without any impact on your Solr integration, or functionality overall.
Of course, migrating from one major Solr Version to another, isn't that straight forward, but, we can even, do that.
However, I can't stress enough that, migration between any Solr minor Versions is not only possible, it's even recommended, since we'll do it all for you, if you want to save time, and money, since, of course, Opensolr also offers FREE membership now.
It has also been a common misconception, that, your Opensolr connection URL returns 404 NOT FOUND, and therefore, it can not be used, and something must be broken.
In your Opensolr Control Panel, you will see your connection url as something like the following:
Now, that URL, will always return an HTTP STATUS 404 NOT FOUND.
Ironically, that means everything is OK.
Your application will use that, as a base connection URL, and append other Solr Request Handlers to it, as you can see in this example:
We have added the /select Request Handler, and that very same connection URL is now responding with a full Solr json format response.
It's an OpenSolr Miracle!