my current contract ends 14/01/2026. when can I change to a lower speed?
14/01/2026
You could try asking, however the only reason for to BT to consider it , would be the acceptance of a new minimum term , so you benefit from a cheaper price ( presumably the reason why you want to reduce speed ) and BT the ‘guarantee’ of you remaining a customer past 14/1/2026
I would hang fire till December
Any changes now would evoke a new 24 month contract.
From December you'll be free to shop around for a better deal.
But to answer your question you have to give at least 30 days notice so 14th December.
The OP gave no indication that they want to change provider, what’s more your advice regarding 30 days notice and therefore indicating the 14th December to serve notice is wrong anyway , unless the OP was quiting a landline service completely and relying on mobile or satellite broadband, then the new provider, even if it’s on a different underlying network serve notice to BT , not the customer.
Although the minimum term date doesn’t change , the date to arrange any migration , should your suggestion be what the OP wants , to minimise any early termination charges or paying a few days at an ‘out of contract’ price isn’t the 14th of December…if the OP did contact a new provider on the 14th Dec, any request to take service from that new provider so far ahead of the minimum term expiration, early termination charges are almost inevitable because the migration could take place much sooner.
If the OP is considering a different provider ( usually for a faster speed rather than a slower one ) contacting the new ISP 10-14 days before the minimum term expiration is the appropriate way to proceed, not arbitrarily service notice yourself a month before the minimum term expires.