I had it put away for a while, since I had cut one of my fingers pretty badly while modifying the original feet of the HG F90 kit... but decided I had better get back to work on it.
Over the past couple of days, I've done some fairly extensive modifications to the legs. Here's an overview of what I've done in recent work:

2- I removed the accessory hardpoint from the leg pieces in order to glue it together as one piece; I'll soon do a lot of putty and sanding work to bring the halves of this piece together.
3- Since the F90 has a recessed area on the inner calf that corresponds with the intake vents on the F91 frame, I cut out the recessed area to expose the vents. I also made the inner calf its own separate piece just like the hardpoint, for the same reasons.
4- The finished inner calf (for the left leg), with the part with the vent on it glued in place. I'll fill the space between the part and the armor with putty once I get some more.
5- The original front lower leg from the old F90 kit. Not much to look at, and not very poseable either...
6- I cut up and trimmed down the inner part of the F91's rear skirt armor in order to attach it to the skirt armor of the F90. It took a lot of effort, but this is one of the parts I'm proudest of.

Next, I plan to cut off the back half of each of the lower shin armor parts that came with the F91, in order to make the "exposed ankle" look of the F90. As for how I'll create the exposed components, I haven't figured that part out yet...
