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On the new domain, XYZ.com 302 redirects to XYZ.com/default.aspx
On the new site, make sure that your 'home' links, wherever they are (breadcrumb, nav, in-page - the LOT) all point to / and then implement a 301 from /default.aspx to /
Then make sure that both /default.aspx and / on the old site point via a 301 to / on the new site.
Tonight all of a sudden XYZ.com has moved down the SERPs drastically and ABC.com is back on the 1st page.
No, this is not the case. It is this way:
ABC.com --302--> ABC.com/default.aspx --301--> XYZ.com --302--> XYZ.com/default.aspx
Sounds like to me the 301 has been lost in translation somewhere