Having a pair of VR-4 Gen1, I looked into the upgrade modifications (including the latest).
On the older website, they listed a modification to bring the older VR's up to the advances of the VR-5/7 series.
This was offered at a unbelievable price of $2500.00 plus freight!! (how times have changed!!)
I've never heard of a modification to the VR-6 specs, but, with all the difference in cabinet design etc, I find it hard to believe it would produce the same sound. That said, I know my dealer has extra parts for all the VR models so a person could do their own DIY if they dared!
Might as well go with the newer VR-5 Anniversary upgrade for the 4's now.
Mike