When it happens. Documenting a relocation, an ownership change or a shift in the business at the time it occurs is cheap and produces a contemporaneous record. Discovering the same facts while preparing a renewal, with nothing written down, is the version that costs money and credibility.
Record it now, argue it later
The rhythm that works is to treat the business file and the immigration file as the same file. When the landlord refuses to assign the lease, write the note that week: what happened, what the new premises are, what equipment moved, what it cost and what effect it is expected to have. When a customer contract is lost or won, keep the correspondence.
When shares change hands, update the register and keep the transfer. None of this takes long while it is happening. The alternative is an interview in which the investor is asked why the business is not at the address on the approved plan and has to explain from memory, with no supporting document, several years after the fact.
Renewals should also be started well before the current admission expires, since assembling statements and payroll for several years takes longer than most investors allow.