VS/VH fare class

Question- further to my Nairobi posting a few weeks ago I booked us into PE through VH- coming up with an H fare code. However, on faffing around on the VS site today I see that the difference for an S fare from a K fare (which seems the same price as the H) is £200- but I have enough miles to upgrade us to UC from an S.
Does Anyone know if VH would be able to change the fare class on payment of a supplement? Or will I have to cancel the flight bits with VH and rebook direct with VS? Silly me for not noticing how to use these miles- but it's only since joining here that I know how to use them properly!!
Thanks
Jason
Does Anyone know if VH would be able to change the fare class on payment of a supplement? Or will I have to cancel the flight bits with VH and rebook direct with VS? Silly me for not noticing how to use these miles- but it's only since joining here that I know how to use them properly!!
Thanks
Jason