Varying an AST

Hi

I need to add some new clauses to the AST that is signed and already started. The tenant has agreed to the changes but I am not sure how to draw this up? Is there some specific working required to ensure the document is valid?

Also, the tenant has a guarantor. What I understand is that the guarantor guarantees the rent/costs etc under the terms of the AST. If I amend the AST, will this variation fall outside the guarantee?

Thanks
Mark