Kafitzner, welcome to the Insider!
That is the big question. And nobody knows for certain how it will play out with the older Fire devices.
There has been speculation, here on the Insider, that there shouldn’t be any changes that would affect the older devices. But who knows, for certain, what changes might occur with future updates involving the operating system. It appears most bet are on a lockout of most side-loading of apps that are not in the App Store. The first, at this point is to turn off any automatic up dates.
Using the magnifying glass, in the upper right hand corner, search for Troy’s link to setup your Fire device so that it can not be updated. I will try and find the link and update my post here. The other alternative is to look at something like the ONN Plus box available at Walmart ($30) which is a Google based OS. The only issue with the Google based systems is that there is rumbling that Google is making changes in who/what apps can be installed on the Google OS devices.