What do you mean by feeling the effects? Are you talking about WMT numbers, or are you thinking something else, like thinking they are migrating data from one storage type to another?
Im thinking that if I were google, at some point for the new system to take over, the old one will have to be switched off as it is unlikely there are the resources to run both concurrantly. So in order to switch I would update all the well seo'd sites on the new system first, as this would be fastest, then no longer update them on the old system (resulting in the position slide) - this is likely to be the most important sites, google, amazon etc plus those that have good seo (wmt members amoungst others). Then the old system would be switch off resulting in resources to recrawl the less well seo'd sites to add them back to the bulk.