Most of us only require a guarantor only if we would not otherwise let to the particular tenant.
Often this is because their income is not at a level I can be sure they will be able to pay the rent - or it may be because of tenants debts or previous judgements against them.
If the tenants circumstances have changed then yes I might release release a guarantor IF the tenant had a sound payment record and and they had enough income to comfortably pay the rent .
BUT the nature of being a guarantor is that they are contractually bound and this is not something they should expect to be released from due to relationship breakdown - or suspicion that it might cost them .
A guarantor is not a referee who offers support - they contractually bind themselves to pay the rent .
Where there is a variation in the rent you should normally either obtain the consent of the tenant AND the guarantor(s) to the agreement OR go the route of a section 13 notice (where the tenant or guarantor have to make an appeal to tribunal if they object)
In most circumstances you can’t unilaterally increase rent without consent or without a section13 notice to all parties (which may be appealed)
That is how i understand it anyway.
You do not have to “release the guarantor” - but if you don’t consult and get their consent OR issue them a section 13 consent it is my understanding they may no longer be held liable by a court for the whole rent but only wat they signed up for ! BUt I’m no expert so please take legal advice on this .