The terms of a secure tenancy may be varied in the following ways, and not otherwise—
References in this section and section 103 to variation include addition and deletion; and for the purposes of this section the conversion of a monthly tenancy into a weekly tenancy, or a weekly tenancy into a monthly tenancy, is a variation of a term of the tenancy, but a variation of the premises let under a tenancy is not.
This section and section 103 do not apply to a term of a tenancy which—
This section and section 103 apply in relation to the terms of a periodic tenancy arising by virtue of section 86 (periodic tenancy arising on termination of a fixed term) as they would have applied to the terms of the first tenancy mentioned in that section had that tenancy been a periodic tenancy.