We were not aware of the fact that bidding was not possible, the site seemed slow but not unusable. Delaying transfers was not necessary.
In the future, we will monitor the site more closely while the data is being copied.
We have to do it now in order to prepare as many things as possible before the migration. It is also necessary because if we would not do it while online, we'd have to prolong downtime to a week or more. Such is the relationship of our current setups data amount/read speed ratio. The new server however is showing some very promising performance. I don't want to jinx it though.
I will add an announcement (message of the day) as you suggested.