“However it seems that I can’t just convert to multisite”
I have no idea why he told you that, because it is possible. I mean,…. it’s kind of the whole point of why multisite was merged in – so existing sites could just turn it on.
User roles on the main site will stay exactly the same.
there may be a couple plugins that don;t like the network, but… if they are used on your main blog, you’re probably fine. If one is broken, there’s likely a replacement.
I mean, multisite is pretty common now, most of the plugin issues are worked out. A good way to check, if it’s the membership plugin you’re worried about is to go to their site and/or support forums and ask them. 😉
The only “catch” right now is this:
You want a subfolder install. In that setup, the main blog will get a /blog/ stuffed in the permalinks to prevent conflicts with site urls and page urls. this can mess up your permalinks (obviously).
But you can ALSO disable it after installation. Go to Super Admin -> Sites and click edit on the main site. Remove it in any of the fields. Presto, problem solved and you now have multisite on your existing blog.
So to answer your last question – just enable multisite. If you’re really nervous, make a backup of your site, install it locally, and do it there as a test run.