Passport requirement for 2year family visit visa

[Cavalier14]

Hey Guys,
My mother in law visiting us every year for the last three years with a 6 month visiting visa and stays for couple of weeks on each visits
This time We suggested her to get a 2year visit visa, so she won't have to go through all the application hassle(TBH all of us), as she's 73yrs old and the application and document staff is not that easy for her to do every year.My question is : does she need a valid passport for whole 2years? Or she can change her passport in the middle of the visa period? her passports expires on May2018.
Thanks IA

[Casa]

The passport can be renewed when required, there's no need for a 2 year validity.

[Cavalier14]

Thanks Casa,So what happens to he visa(which is attached to the passport) when passport expires and she has to get a new one?
Does she need to send her new passport to UK embassy to change a visa and transfer it to the new passport or she could travel with two passports in hand?

[vinny]

You don’t have to transfer your visa - you can carry both your old and new passports when travelling to or from the UK instead.

[Cavalier14]

Thanks a lot Vinny, that's a good news,I appreciate your help.