The new carries and bolts will work with the older slides, no problem. You just won't make use of the extra notch.
If you want to put an old carrier onto a newer carrier (i.e. action bars), you would either have to file off the superfluous tab on the left action bar, or file a notch into the older slide. Very doable.
The Flexi-Tab system includes the elevator and a new bolt and slide. A flexi tab elevator will work without the new bolt and slide but will not give you the flexi tab benefits.
Flexi-tab slides have milled relief cuts on either side at the back of the slide. Flexi-tab compatible slides were made for both styles of carriers (AKA action bars).
So bottom line, you can buy a brand new carrier (AKA action bars) if you want, but you will need to file a notch into your older slide, or replace the slide with a new type (assuming you have an older slide with only one notch on each side).