Tenant's girlfriend will move in

Other

My tenant is on a 12 months AST which will end in August this year, i.e. just more than 6 months to go. Both he and I would like to sign a new tenancy agreement to have his girlfriend as joint tenant with him. I have a few questions regarding the formalities:
1. What happens to the old tenancy in this case. Do we need to have a 'surrender, or deed of assignment?
2. What is the term of the new tenancy, i.e. to end on the same day as the previous tenancy, or minimum six months, or can it be for another year
3. Do we need to go through the whole deposit formalities again, i.e. take money out and put it back in, giving the tenants required form etc.? We use the RLA's DepositGuard.
4. Do we need to sign a new inventory? We are not providing any new furniture.
5. In light of the above, to save time and hassle, can we sign a deed of variation instead, adding his girlfriend as joint tenant? That way, I presume this deed will be the only document we need to sign? If so, do RLA have a deed of variation template I can use?

Many thanks and I look forward to receiving your advice as soon as possible.

Catie