UPN Suffixe and Forest Trust (same domain name)

Hi,

We have 2 domains : 1 old (let's call it domainold.local) and 1 new (let's say newdomain.local) with a forest trust between them. Users are migrating to the new domain.

We would like to set an alternative UPN suffixe in the newdomain (newdomain.local) using the old domain name (domainold.local) to allow some users to logon with it. Note : the users have a new sam name in the new domain.

But we already have declared a trust with this same DNS name (domainold.local) between the two forests.

So the question is : Can I add the alternative UPN "domainold.local" in "newdomain.local" if we have an active trust with "domainold.local" ? I need to be sure adding the UPN will not interfere with the trust or will not block any use of the old domain accounts from the new domain.

 Hope I'm clear enough :) Thanks.