Val Thorens early season

My daughter is planning to go to VT with Uni on the 10th December which is
the ssecond week of the season.

There is an option of a 3 Valleys pass on top of the standard VT area pass
for £49. On the VT site the difference is around 20 euro's for that time, so
to save her some money does anyone know if you can upgrade on a daily basis
or at a kiosk?

I don't think it is worth buying the full pass now as I'm not sure how much
will be open even with good snow but I don't want her to miss out as much as
I don't want her to waste her limited funds.

thanks......Mike