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We are going to use a 301 redirect across the whole site, and understand there is likely to be a drop in rankings.
What we are worried about is redirecting such a large number of pages ot a new domain. Will the new domain be penalised becuase it suddenly has 20K pages? Should we redirect things more slowly i.e. a section at a time?
That still doesn't remove the so-called sandbox period of getting a "trust check-up" for your new domain. But there should be no worries about getting a true penalty. There are many reasons for lower ranking that are not really penalties, and a new domain is one of them.