I can't answer the question about how to transfer accounts between DreamHost accounts - you'll need DreamHost for that, but there won't be any negative consequences resulting from this transfer. Actually, I should qualify that - there is a chance that there may be some downtime, but it should be brief.
After the transfer is complete and your sites are functioning as they normally would, I see no reason for there to be averse consequences. Unless you've changed the URL structure or what not, there will be no averse SEO effects. Search engines will continue to visit the site and resume indexing the content, provided that the links have not changed.