I’m glad to hear that you believe the Designer series of products are well suited for the purpose intended. I.e. aimed at those engineers who need access to low cost, easy to use schematic & PCB design tools with ready access to up to date part data.
There is a migration path to the rest of the Mentor PCB tools, what flow is your company using PADS or Xpedition Enterprise tools, we have two primary PCB flows.? I.e. what is the “full Mentor Graphics package”?
Either way, there is a direct migration to PADS, just open those Designer files in a PADS based solution. As you are probably well aware, the Designer tools are based on the PADS flow.
If you need to be able to Migrate to Xpedition Enterprise the route is still to go through PADS first. So if you do not have a PADS seat you could use a local service bureau to migrate those designs for you.
Hope this helps.