Paul, I've worked on many of those directories over the years. I've pulled them totally apart, rebuilt them, and changed the strip sizes in them. Nelson Harkins has different size strips, so you can replace the smaller ones with larger ones if they fit. If you need them to be longer to accommodate larger text, you can do that to, it just takes a little effort to work through it.
The back pieces come out really easily. You just lift the bottom, pull it out an inch or so and then it'll drop completely out. Once out, you'll see it's not much to it. They are made from a piece of white acrylic with a black plastic piece on each side that allows for the strips to slide in and it not show any light.
You can, with some effort, get those black pieces off most of the time. It might take some work, but you can normally work on them and get the strips off. Once off, you can make the new back piece from white acrylic, then just glue the black side pieces on and you'll have your new backs that would accept longer, taller strips.
Those directories work great when someone knows what they are doing. When they are updated by morons, they end up being a heck of a mess. People don't seem to understand the way they work with the "Z" strips, so they make one strip with the tab up, one with the tab down, and butt them together and you get a light leak. Then some idiotic maintenance guy steps in to save the day with electrical tape. Then you have the property manager that thinks they can get them done cheaper somewhere else, so they switch vendors, the new vendor doesn't know how to match fonts or margins, so the new strips have different fonts and the left margin is off by 1/4".
We maintain two for large buildings that literally have 300 strips in them. Only 20 of them are engraved. Who thought they'd have 300 tenants at any time?????
Sorry......yes......I know about those directories more than I wish I did