You can still use the CRM 4 endpoints; one thing I'm sure of is whether the CrmImpersonator class will still work.
The decision about upgrading code can be argued either way. As MS provide backward compatibility for 1 major version, then it is viable to only rewrite your code for every other version upgrade
Microsoft CRM MVP - http://mscrmuk.blogspot.com/ http://www.excitation.co.uk