Replacing old tenacy agreements with new ones

Tenancy Types and Management

I am a recent "joiner" to RLA and I am aware that much of the tenancy documentation I have been using over the past few years is now dangerously out of date. I therefore intend to ask all my tenants to re-sign the new RLA documents and have their guarantors sign the Unlimited Deed that I would need to rely on in the event of default by the tenant. (My current Guarantor form is not executed as a deed and I am unsure whether it can be relied upon.)

My question is: Is there a process or procedure that I must follow to have the new documentation signed (e.g. do I need to serve notice to terminate the old tenancy) or can I simply contact the tenants (and their guarantors) and ask them to sign the new tenancy paperwork? (I have no problem in them each having a new AST for at least 6 months - they're all lomg term tenants anyway.)

If, for whatever reason, a tenant refuses to sign the new paperwork, I presume I would need to serve s21 to terminate in 2 months time but at least I'd be pre-empting any future disputes where the strength of my current tenancy documentation might not hold up.