Exactly what i've done with my Russians which are based 6 x 15mm figures per 30x30mm base.
My Bavarians i'm rebasing to 8 figures on 40x30mm. Our guys are doing 40mmBW with either 6 or 8 figures per base
With true 15mm figures, 30mm BW starts to look a little sparse. But with these new "large" 15mm, which are true 18mm or 19mm figures, the 30mm BW works okay.
For me, visually, I think six "large" 15mm figures on a 30mm x 24mm looks good. Putting six figures on a base with a 40mm BW looks a bit sparse to me, and more like an irregular unit. But that's me. I tend to prefer the look of the "closed" ranks. For irregular formations in Lasalle, four and five figures on a 40mm BW, or three and four on a 30mm BW has a nice look to it.
I did paint up some "true" Lasalle units. That is, eight figures on a 40mm by 25mm base. Heck, if I could paint faster and had vast resources of figures, I would do bases of ten figures (40 figure units). Yes, you can fit ten figures (two rows of five) on a 40mm x 25mm base. But again, depends on the figure's pose and actual size. Very easy with true 15mm figures, and it can be done with the less heftier true 18mm figure, but starts to get a bit snug with some of the heftier true 18mm figures.