Yanmar have used both ATF and SAE30/eng oil in their gearboxes. General rule: disk-clutch=ATF, cone-clutch SAE30/engine oil.
Main reason is the breaking of the oil-film by the clutch-system to obtain the friction/engagement of the clutches. The film-thickness in the SAE30 is more difficult to breake for the plate-clutch, the cone clutch engages with much more force and less friction surface and should manage if all is in order.
The following link might be a help:
There is no big problem to change the oil, but You should do at least 2 changes to wash out the old oil. The reason for the old owner to have ATF might be the symptom You describe as 'delayed shifting'. This might encrease with the use of SAE 30 oil. Whatever You do, do not go syntethic.