As long as you migrate the user and maintain their SID between the NT4
domain and AD you should be fine. IF you don't maintain the SID then you
will have to re-apply the security into Sharepoint as Sharepoint keeps
track of the user SID

We only have four users on the portal  and they are Administrators. I have already added the users from the AD Domain so that there is an NT4 user and the corresponding AD user. The userids are different: NT4 user is 8 characters, first seven of lastname plus first initial; AD user is firstname.lastname up to 20 characters.

I'm concerned about SharePoint breaking like it did when we apply the last SP and had to completely reinstall and rebuild the portal site.

We recently did an NT4 to A/D Migration and this is what I had to do.

1. Backup the portal(s)
2. Migrate the machine over to the A/D
3. Reboot the box
4. Remove Sharepoint, reboot, Re-install Sharepoint
5. Restore the portal(s)
6. Go into Central Admin and I forget which Option I had to uncheck/recheck so that the Index rebuilt

Go to central admin. You need to crawl the AD. Usually
AD Users profile will be migrated to Sharepoint SQL
Server. Once done you can see all the list of users
from the Central Admin page.

We removed the server  from the NT domain and then added it to the AD. Portal comes up fine. The only down side is that the users who logon with the NT credentials they see there MySite that they created previously. If they logon with their AD credentials they get a new MySite. If my memory servers me correctly I think someone wrote a tip about copying/moving libraries. I'll let y'all know what I find out.