I presume they would be available just as they are now from the drive.
That depends on what you mean by “available.”
When you migrate everything to cloud Lightroom, you don’t lose your current set of images that you’ve been storing locally, because what gets uploaded are copies. Those uploaded copies in the cloud become your working originals. In other words, you do a fundamental switch from working from your locally stored images to working from the Lightroom photos cloud server. This could free up local storage, because you could move your current originals off the computer. Then the only working images actually on your computer would be the ones temporarily downloaded and cached while you’re editing them. And, all your images are always available to any device you own that can run a cloud Lightroom app (Mac, Windows, iOS, or Android), as long as you have an Internet connection.
You don’t always have to work from the cloud. Cloud Lightroom now has a Local tab so that you can browse and edit images without having to upload them. However, be aware that the editing features available in the Local tab are much more restricted than both cloud Lightroom editing and local editing in Lightroom Classic. If local editing with a full feature set is more of a priority for you, you should stay with Lightroom Classic.
Think carefully about what you like the most about Lightroom Classic, because in addition to your source files moving online, cloud Lightroom has a different feature set than Lightroom Classic. Some features are better than Classic (like automatic keywords), some are worse (like manual keywords), and some are missing (like printing).
One big question is: How important is it to preserve how you have organized your photos in Mac/Windows folders? Because when you migrate to cloud Lightroom, that permanently disconnects the cloud copies from any folder organization you may have used in Lightroom Classic, and it cannot be restored even if you do a bulk download. All organization becomes virtual, similar to using Collections in Lightroom Classic. In part this is because virtual organization makes it a lot easier to preserve organization across different desktop and mobile devices, but this aspect of migrating all your images to cloud Lightroom means you have to be OK with losing your current folder organization. For some people this is no problem, for others this is a deal-breaker.