I'm about to install a wildcard SSL cert *.example.com (from digicert) over top of an existing single domain cert www.example.com (from thwate).
Are there any potential problems to watch out for when doing this? Any special steps required?
In my experience installing a number of Thawte wildcards, they cause no problems with any modern browsers - you shouldn't have any problems. Watch out for your Intermediate bundle, be sure to change it from Thawte's to Digicert's while putting in the new cert.