The only way for you to transfer them to family visit is by exiting the country, and coming back using the Family Visit Visa. there is no such transfer, but you can transfer it into "husband or Father" sponsorship. Visit visa, they need to exit, have their existing visit visa be cancelled, and enter with the new one. this way, they have maximum stay of 6 months.

It's not because things are difficult that we don't dare, it's because we don't dare that makes things difficult!