Va looks at several things but one of the main drivers is the intersection of limit load factor and the stall line.
If, say with a mod, the stall speed reduces, then one would expect the Va to reduce on the new stall line. Depending on the two lines, a modest weight increase post-mod may still result in a lower Va.
One presumes that there is no design change to limit load factor ..
So what is the pre-mod and post-mod stall speed variation ?