Buying a house with an existing tenant

Tenancy Types and Management

We're in the process of buying a house with an existing tenant whom met her when we viewed the property. The owner (who lives down South) has had her in the property for 11 years and there is an AST from when she first moved in. It has not been updated since then although the rent has gone up a bit. Apparently the owner has a good relationship with her and things have been kept friendly and informal. When we met her we agreed that if we bought the house we'd potentially do some work that she's requested (splitting a bedroom). We also explained that we would need to put her onto an AST with us, that she would need to pay a deposit (there is none at the moment) and that the rent would go up after 6 months as we'd be doing this work that she's very keen for us to do. She agreed with this and it's been confirmed in writing via the Estate Agent. My question is at what stage are we able to put her onto our AST - do we do it on the day we complete or can we have her sign it beforehand with the completion date as the start date? Before that, we'd also like to check her bank statements for the last 3 months or do a credit check, again to reinforce the idea that we do this with all our tenants (new AST, deposit and check). Our solicitor has said that she'd need to agree to this which is completely understandable. However, I'm keen for the new AST to be in place as soon as we complete and to communicate that the deposit and rent will be due on that day.