This “conversion” is so easy that I really hesitate to call it a conversion. Dark Age, Hordes, Warmachine, and Malifaux among other games all use 30mm circular bases for their human-sized figures. My understanding is that Dark Age used these first and Privateer Press adopted them. Games Workshop uses its 25mm round bases for Warhammer 40k, Necromunda, and most of its sci-fi games no longer in print. These circular bases were also supplied for some of GW’s fantasy dungeon delving games.
Guess what? You can mount a 25mm round base onto a 30mm round base and it doesnot look all that terrible. Now, you could just glue the 25mm base onto the 30mm base, but why restrict yourself to only using the attached miniature in only one game’s system? You could also just leave the 30mm base unattached and pick up both miniature and its dual bases for games that use the larger base. This can lead to the larger base being left behind and wasting precious seconds restacking your miniature.
My solution has been to add small neodymium magnets to the underside of the 25mm base and to the top of the 30mm base. You could probably get away with just one on each base, allowing for some pivoting of the figure, but by gluing or green stuffing two magnets on each base, you get a nice solid magnetic lock with no pivoting for minimal expense. The cost of the magnets should be a lot cheaper than the cost of buying a new model as well as painting it. I buy my magnets from K & J Magnetics.
Yes, your new double-based figure is now about 3mm taller! If this is a terrible shock to your sensibilities and you think this provides some odd advantage for Line of Site while not making the troop easier to spot itself, enjoy your shock, while I enjoy my miniature in multiple game systems. Also you might enjoy the fact that your 25mm or 28mm miniature now is closer to 30mm itself.