Altering length of the term on an AST

Tenancy Types and Management

A tenant, an elderly lady has just moved into a flat of mine. I used a letting agent to find and move the tenant in and sort out all the paperwork, deposit, inventory etc etc, but I self manage after the move ins..

The tenant wanted a 6 month AST which is what she signed, but now two weeks later she has told me she loves the place so much that she wants a longer contract to give her long term peace of mind which I am more than happy to do.

My question is can we simply cross out the dates and amend with our signature next to the amendment as you would do with a mistake on a cheque for example, or do I need to rip up the document and start again? What implications does this have on everything else such as the protected deposit and all the other documents that need serving at the time of a new tenancy?

Also is there a maximum term length for an AST?

Thanks again.

David.