Hello wise internet folks. I have been living in a 2 room flat for over 15 years. The whole building I live in is owned by one landlord and suddenly an opportunity has presented itself where by my neighbour's larger flat is about to become available.

I hopefully will get first dibs at taking over the contract. I would be comfortable spending next 15+ years there so could do with some help.

Q1- If I take over the contract does the rent stay same? Neighbour pays same rent as what I pay for my smaller flat.
Q2- Am I right in assuming that I have to find someone for my flat?
Q3- Do I have to go through the headache of proving my income again or does being a tenant for last 15 years and paying same rent take care of that?
Q4- Who should approach the landlord? Me or the neighbour?
Q5- How would the whole deposit situation work?

Any additional advice you could give me so that landlord does not shoot down my plan of moving to the neighbour's flat? I most likely will have to pay rent for 2 flats for couple of months but it might be worth it in the end. Thank you for your time and patience.

A1- It may only stay the same if you take over the existing tenants contract. This is normally only possible if the existing tenant canceled the contract outside the normal notice period. Otherwise the landlord can adjust the rent to the local market, and my to some improvements first before renting the place. Plan to pay a slightly higher rent than the current tenant does.
A2- If you cancel your contract outside of the notice period yes, otherwise no. Check your contract about the dates when you can cancel.
A3- Normally not, as the landlord already knows you. If you never missed a rent than you good.
IMHO, you should first contact your neighbor. Best if he is your friend and supports your cause. Then contact the landlord together.
Its up to the landlord, but it shall not be more than three months rent at beginning of the tenancy.
Depends on who your landlord is, a company or a private individual. How good to you know the guy?