It depends on what type of payment you have made for your Bronze membership.
IF you paid for a whole year at once, then the balance left (it is pro-rated to the time of renewal based on an amount for each month) will be applied to the upgrade, which will be activated immediately from the date of purchase and be good for one year from that date.
IF you pay by the month, you WILL HAVE TO CANCEL the current subscription via Pay Pal, then do your new subscription.

I hope this helps, if not ask any scribe or demi for more help.