If I start making a drawing in meters units and I want/need to switch the units to mm how can I do that? is it possible?

I tried to do just that. I syarted drawing in meters units by clicking on Format/Unit systems and changing the Block unit scale to mm. But nothing seemed to happen

I immagined that the dimensions would for example start to have 3 more 000: eg if a line was 3 m long initially, now it would show as 3000 mm long

Does it make sense?