AFAIK those that were given a HUB4 instead of a HUB5 when they ordered the GIG1 package past couple of months (NEW customers) will be contacted by VM when HUB5's are in plentiful stock again. A VM forum staff member had already said that on these forums.
For existing customers, I am afraid it's likely you'll have to stamp your foot up and down with retentions to try and get one if you ring in about cancelling because of price increase but a HUB5 may keep you in the deal. For a brief time, somebody posted a web link to a free upgrade, but then when it got posted on hotukdeals.com, too many people jumped at it (often people on lower tiers than GIG1) and VM closed the link down. If they are still low in stock however, you might just hit the brick wall.
Virgin Media's Own Press Release:
As part of a phased roll-out, Virgin Media will offer the Hub 5 to existing customers first at no extra cost.
It goes on to say about a trial period where invited customers were for that trial.