XF 2.3 User Upgrades - What happens when changing tiers? (how to customize)

[BubbaLovesCheese]

Assume there are 2 tiers:

Silver $5.00 for 6 months
Gold $10.00 for 6 months

1. If a Silver member with 3 months remaining purchases another Silver, will they now have 9 months on their membership?
   • Does this happen automatically within Xenforo?
2. If a Silver member with 3 months remaining purchases Gold, what is the result?
   • Can we replace their Silver membership with Gold? (ie, remove from "silver" group and add to gold?)
   • Can we pro-rate their Gold membership with the remaining time/value of their Silver?

If not, what would be the best place in the flow to add the necessary code to make it happen?

Thanks.