Changing SLD is a 'bigger' infrastructure project. It is not a messaging project, like the simple upgrade of Exchange.
And all existing solutions (migrate forest or even a future rename tool w/ E2K7), for a medium-large company,
will be done in a large amount of time (form serval months
to year).
The pratic effect is to stop the upgrade from E5.5 or E2K,
for a 'dangerous' period. Because we can not use new anti-spam/antivirus integrated features. And all the new 'corporate policies' with Transport Rules. And the new mobile functions. Depending the case and urgency, similar products of other companies will be evaluated to make these funcions.
Btw:
IPV6 ist not a good reason for the real world. If it is incompatibile with SLD, them it is better to block the use of IPV6 on SLD. And not SLD by SLD itself.