Schema changes can actually be reversed, after all AD is based on LDAP. However, Microsoft prevents schema changes from being reversed.
Yes, how true and how silly. There are some architectural decisions that were made by some (now) Microsoft millionaires that never made sense to me. This is one of the biggies.
I've seen so many customers our there run into schema problems. Worse, you still find many customers out there that are just simply afraid to extend their schema. Either way, test your schema, read Microsoft's guidance on the topic and consider a product like "Recovery Manager for Active Directory Forest Edition" which can protect you from that potential career limiting mistake.
...one of our clients recently ran into a problem attempting to test the OCS schema update (yes, notice I used the word test). While performing the test, in a lab environment, the update failed with a conflicting LinkID error. After researching the issue, we found that another previous schema update (from a well known software vendor whose name I shall not mention) used a LinkID that was reserved for Microsoft (or maybe it was the other way around, we are still looking into this). In other words, I would even scrutinize schema updates that come from well known sources, this includes Microsoft.
Definitely a best practice - test your schema update even if it is from Microsoft before updating your production forest.
Microsoft, Active Directory, Quest Software