During 4th edition, it is not uncommon to have a heavy weapons team like the missile launcher team, to have the loader and the shooter based separately. With 6th edition however, they need to be based on a single large base. Here is a simple way to make your miniatures current (if they are base separately), as well as avoiding the pains of ripping your existing miniatures from their original 28mm bases.
Glue the foam board to the large base (I used the old thin large base. This technique is also applicable to the new larger bases. This will no doubt make your overall miniatures stand taller). You can score the foam board using the stanley knife and a pencil to create a tile pattern or just leave as is. Texture and paint the base to finish off.
Now you have an interchangeable large base that fits two existing 28mm based miniature.
As always good luck in your scratch building