A similar question was asked and answered on Ubuntu forums the other day regarding upgrading from 11.04 to 12.04 without going through 11.10 first. So, I will refer to the excellent answer provided by Kansusnoob here
Link to a previous answer on another site
Note this comment by kansusnoob:
There are now three ways to upgrade:
1: Using the update manager
2: Using the alternate CD
3: Using the live CD
Note how he proves, by providing this image, that the live CD can be used to upgrade an existing install to a distribution release and even do it with 11.04:
Image of live CD Installation type options