There's nothing in the Scrum Guide that says that estimation needs to be done in points. There's also nothing that says that items in the Product Backlog or Sprint Backlog must be user stories.
If your team is doing well at planning and executing their Sprints while estimating the items in the backlog in hours or days, why do you feel the need to change that?
You state that this will "mess up their velocity calculations, backlog grooming, release planning" as well as giving an incorrect picture to the Product Owner and leads regarding current status. Let's look at this one at a time.
First up, velocity calculations. Velocity is not a part of Scrum. The Scrum Guide makes no mention of the word "velocity" at all. Velocity is a common measurement, but you can still compute it with hours. Velocity is simply the amount of work completed in a specific amount of time. Often, this is Story Points per Sprint, but it doesn't have to be. It could be "hours of value added effort per Sprint" just as easily. If you are estimating Backlog Items in hours and know how many hours are in a Sprint (number of people * number of working days * working hours per day), you can divide "task hours" by "total hours in a Sprint" to get your velocity. In a perfect world, every working hour will be spent on value-added work. But that's not always true, so your velocity can be a predictor of how much time your team spends working on Backlog Items.
Second up, backlog grooming. In the Scrum Guide, this is now called "refinement" or "Product Backlog Refinement". It is simply the act of adding details, estimates, and order to the Product Backlog. The units that you use for estimation have no bearing on the act of Refinement. After Refinement, you should have Product Backlog Items that are sufficiently detailed for the Development Team to work on with an estimate attached to them and properly ordered based on the current understanding so they can be pulled into a Sprint.
Next, release planning. If you have a properly ordered Product Backlog with estimates attached to each item and you know your velocity, release planning should be easy. You should be able to pull off the top n items until the total estimate equals your velocity that deliver that in the next Sprint, accounting for any variances (people taking vacations or sick time, holidays, any kind of work events not tied to the items in the Sprint, etc.).
Finally, burn down. If you're measuring in estimated hours and are tracking hours left in the Sprint, you can still have a good idea of burn down and look at it every Daily Scrum or make it visible in your tooling. Regardless of what your estimation units are, you should be at 0 at the end of your Sprint. Burn down is just a graphical representation of how you are completing your work in your Timebox.
Now, just because you can use something other than points doesn't mean that it's necessarily a good idea. There are plenty of posts about the advantages to using points - see this question on Software Engineering Stack Exchange, this question on Project Management Stack Exchange, this post by Scrum, Inc., this post by Mountain Goat Software, and this post on Agile Buddha. However, estimating in hours is not a problem to doing Scrum, especially if it is working for your team.
Consider the principles behind the Agile Manifesto and the Scrum Values. If your team claims to be agile, they should be adhering to the principles behind the Agile Manifesto in whatever implementation works best for them. If your team is claiming to be following Scrum, they should be following the Scrum Guide and the Scrum Values regardless of their implementation of each of the activities. If your team is not following these principles, you can work to guide your team to them, but the team needs to figure out what their method of following these principles works best for them. You can share your thoughts, but not impose your will on the team.