Amending Rent After Issuing Section 13: Advice

I have recently issued section 13. Having spoken to one of the tenants, I am minded to delay the start of the new rent for two or three months. If I agree to this (by email), will section 13 still be valid, or will it need to be amended?  

I would prefer to leave the S13 and hope to amend the rent again in a year, but I don't know if that would be permissible.  

Advice appreciated.

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