The agreement with the landlord consists of the lease/rental contract, the law, and other unspoken expectations of both parties.
Some times more things are spelled out (lease contracts that have built-in price changes; for example I've seen ones that basically auto-renew every year, with an optional x% increase on the landlord's side, which usually but not always is taken), sometimes they aren't.
You can even agree to things that aren't allowed by law; both sides just never bother reporting it (this is one of those unwritten things you find from time to time).
Usually a tenant "unilaterally changing the agreement" is just falling back on the tenant protection laws to their advantage. Savvy landlords know the laws and how to work with them, inexperienced landlords get caught out (often because all the tenants before have never used them).
Some times more things are spelled out (lease contracts that have built-in price changes; for example I've seen ones that basically auto-renew every year, with an optional x% increase on the landlord's side, which usually but not always is taken), sometimes they aren't.
You can even agree to things that aren't allowed by law; both sides just never bother reporting it (this is one of those unwritten things you find from time to time).
Usually a tenant "unilaterally changing the agreement" is just falling back on the tenant protection laws to their advantage. Savvy landlords know the laws and how to work with them, inexperienced landlords get caught out (often because all the tenants before have never used them).