Whenever a landlord has the benefit of a surety guaranteeing (for example) the tenant's obligations in a lease, it should always be wary of doing anything which could release the surety from its liabilities under that guarantee.
Such a release can occur inadvertently if there is any variation to the terms of the lease unless the surety has given its formal written consent to that variation.
Whilst there may be arguments about the nature of a variation which might trigger a release, it is best not to give a guarantor the chance to raise any arguments in the first place. The way to do that is to join the guarantor as a party to every written document which relates to the lease, even those which record changes which you know the guarantor is aware of or has agreed to. Failure to do so may let the guarantor off the hook.