It wasn't me as dawnofwar has said.
I'd also point out that really tenants moving to new agents (or even staying with the new GDH - Turner PMS) do not actually need to get new tenancy agreements drawn up under normal circumstances as the agreement is between the landlord and tenant, not the agent and tenant (though the agent will be mentioned within the agreement) things such as new bank details for rent payments and new agent details can usually be issued to the tenant in written form which would count as an update to their agreement and this is normally legally acceptable, however if your deposit is with TDS you WILL need a new tenancy agreement. As has been pointed out normally the assured shorthold tenancy agreement (AST) has a fixed term of 6 months as standard and some landlords/tenants may not want to be forced into another 6 months at the property however, as I added to my link on the info for getting deposits back, there is no minimum fixed term an AST is required to have. The tenant does however have a right to remain at the property for at least 6 months. this basically means that you could have an AST with no fixed term period (it begins as a periodic tenancy and just rolls over each month until someone gives notice) so the tenant can give one month's notice at any time, the landlord however cannot give the tenant a section 21 notice to vacate the property which expires before the 6 months is up (the landlord can give the tenant notice to leave at the 6 month period at the earliest). And landlord must also give the tenant at least two month's notice (though more can be given if wished). If the landlord wants the tenant out of the property before the end of the initial 6 months then they must have reasonable grounds for this such as lack of rent or damage to the property caused by the tenant (and these grounds need to be included in the AST).
Hope that was clear! It's a bit technical and I have a tendency to ramble once I get into legal jargon!