如果房子的价值上升了，如何改mortgage，或者说remortgage?

我听说如果房子的价值上升了，最好是重新设置mortgage比较划算。这个道理是怎么一回事呢？
举个例子：比如，买的时候，房子的价格是10万（贷款5万）；现在的市场价值是15万（现在贷款还剩下4万）。那么该如何重新设置mortgage才有利？

无论如何你都是和银行贷款了5万，你把这笔钱还上就行，和你的房价怎么变化都没有关系

It's called equity release as the equity which is current value of your property minus outstanding balance of your mortgage has increased. It was quite popular a few years ago but gradually gained a bad reputation because it is considered ricky as property price may go up and down and you may end up with negative equity if price goes down significantly. Also in current market condition most lenders are very cautious about this sort of things and they may value your property still low so that you won't have much to release.